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Logging
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#database-profiling" class="md-nav__link">
<span class="md-ellipsis">
Database Profiling
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#authentication" class="md-nav__link">
<span class="md-ellipsis">
Authentication
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#authorization-role-based-access-control-rbac" class="md-nav__link">
<span class="md-ellipsis">
Authorization: Role Based Access Control (RBAC)
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#replica-set" class="md-nav__link">
<span class="md-ellipsis">
Replica set
</span>
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<li class="md-nav__item">
<a href="#sharding" class="md-nav__link">
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Sharding
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<li class="md-nav__item">
<a href="#aggregation-framework" class="md-nav__link">
<span class="md-ellipsis">
Aggregation Framework
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<nav class="md-nav" aria-label="Aggregation Framework">
<ul class="md-nav__list">
<li class="md-nav__item">
<a href="#variables" class="md-nav__link">
<span class="md-ellipsis">
Variables
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#match-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$match Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#project-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$project Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#addfields-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$addFields Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#group-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$group Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#unwind-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$unwind Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#count-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$count Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#sort-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$sort Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#skip-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$skip Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#limit-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$limit Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#lookup-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$lookup Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#graphlookup-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$graphLookup Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#sortbycount-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$sortByCount Aggregation Stage
</span>
</a>
</li>
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</ul>
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</ul>
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<span class="md-ellipsis">
HTML
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<a href="../languages/markdown.html" class="md-nav__link">
<span class="md-ellipsis">
Markdown
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Pandas
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Seaborn
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Requests
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Beatiful Soup
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C#
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C#
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<a href="../languages/dotnet/csharp/linq.html" class="md-nav__link">
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Linq
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Collections
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Reactive Extensions
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<a href="../languages/dotnet/csharp/async-programming.html" class="md-nav__link">
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Async Programming
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Unit Tests
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ASP.NET
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Minimal API
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FIlters
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Middleware
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Razor Syntax
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<a href="../languages/dotnet/asp.net/blazor.html" class="md-nav__link">
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Blazor
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SignalR
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Database
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ADO.NET
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Entity Framework
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Java Collection Framework
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Javascript
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DOM
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jQuery
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React
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React
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React Router
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React Tests
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Redux
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PowerShell
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Scripting
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Bash
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Bash
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Scripting
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Commands
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Misc
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Misc
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<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../misc/git.html" class="md-nav__link">
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Git
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<a href="../misc/graph-ql.html" class="md-nav__link">
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GraphQL
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<a href="../misc/regular-expressions.html" class="md-nav__link">
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RegEx
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SSH
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WebComponents
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Linux
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Linux
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Filesystem
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Filesystem
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File Links
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/proc
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/sys
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System Init
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System Init
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Systemd
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OpenRC
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Table of contents
</label>
<ul class="md-nav__list" data-md-component="toc" data-md-scrollfix>
<li class="md-nav__item">
<a href="#data-types" class="md-nav__link">
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Data Types
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</a>
</li>
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<a href="#databases-collections-usage" class="md-nav__link">
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Databases &amp; Collections Usage
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<a href="#operators-mql-syntax" class="md-nav__link">
<span class="md-ellipsis">
Operators (MQL Syntax)
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<ul class="md-nav__list">
<li class="md-nav__item">
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Expressive Query Operator
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</ul>
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<li class="md-nav__item">
<a href="#mongo-query-language-mql" class="md-nav__link">
<span class="md-ellipsis">
Mongo Query Language (MQL)
</span>
</a>
<nav class="md-nav" aria-label="Mongo Query Language (MQL)">
<ul class="md-nav__list">
<li class="md-nav__item">
<a href="#insertion" class="md-nav__link">
<span class="md-ellipsis">
Insertion
</span>
</a>
</li>
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<a href="#querying" class="md-nav__link">
<span class="md-ellipsis">
Querying
</span>
</a>
</li>
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<a href="#updating" class="md-nav__link">
<span class="md-ellipsis">
Updating
</span>
</a>
</li>
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Deletion
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</li>
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<a href="#mongodb-database-tools" class="md-nav__link">
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MongoDB Database Tools
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</a>
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<ul class="md-nav__list">
<li class="md-nav__item">
<a href="#mongoimport" class="md-nav__link">
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Mongoimport
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#mongoexport" class="md-nav__link">
<span class="md-ellipsis">
Mongoexport
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#mongodump-mongorestore" class="md-nav__link">
<span class="md-ellipsis">
Mongodump &amp; Mongorestore
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</a>
</li>
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<a href="#indexes" class="md-nav__link">
<span class="md-ellipsis">
Indexes
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</a>
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<ul class="md-nav__list">
<li class="md-nav__item">
<a href="#index-types" class="md-nav__link">
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Index Types
</span>
</a>
</li>
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<a href="#diagnosis-and-query-planning" class="md-nav__link">
<span class="md-ellipsis">
Diagnosis and query planning
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#index-creation" class="md-nav__link">
<span class="md-ellipsis">
Index Creation
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#index-management" class="md-nav__link">
<span class="md-ellipsis">
Index Management
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</li>
</ul>
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<li class="md-nav__item">
<a href="#cluster-administration" class="md-nav__link">
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Cluster Administration
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</a>
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<li class="md-nav__item">
<a href="#mongod" class="md-nav__link">
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mongod
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</a>
</li>
<li class="md-nav__item">
<a href="#basic-shell-helpers" class="md-nav__link">
<span class="md-ellipsis">
Basic Shell Helpers
</span>
</a>
</li>
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<a href="#logging" class="md-nav__link">
<span class="md-ellipsis">
Logging
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#database-profiling" class="md-nav__link">
<span class="md-ellipsis">
Database Profiling
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</a>
</li>
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<a href="#authentication" class="md-nav__link">
<span class="md-ellipsis">
Authentication
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</a>
</li>
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<a href="#authorization-role-based-access-control-rbac" class="md-nav__link">
<span class="md-ellipsis">
Authorization: Role Based Access Control (RBAC)
</span>
</a>
</li>
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<a href="#replica-set" class="md-nav__link">
<span class="md-ellipsis">
Replica set
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</a>
</li>
<li class="md-nav__item">
<a href="#sharding" class="md-nav__link">
<span class="md-ellipsis">
Sharding
</span>
</a>
</li>
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<li class="md-nav__item">
<a href="#aggregation-framework" class="md-nav__link">
<span class="md-ellipsis">
Aggregation Framework
</span>
</a>
<nav class="md-nav" aria-label="Aggregation Framework">
<ul class="md-nav__list">
<li class="md-nav__item">
<a href="#variables" class="md-nav__link">
<span class="md-ellipsis">
Variables
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#match-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$match Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#project-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$project Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#addfields-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$addFields Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#group-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$group Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#unwind-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$unwind Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#count-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$count Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#sort-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$sort Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#skip-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$skip Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#limit-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$limit Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#lookup-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$lookup Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#graphlookup-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$graphLookup Aggregation Stage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#sortbycount-aggregation-stage" class="md-nav__link">
<span class="md-ellipsis">
$sortByCount Aggregation Stage
</span>
</a>
</li>
</ul>
</nav>
</li>
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</nav>
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<div class="md-content" data-md-component="content">
<article class="md-content__inner md-typeset">
<h1 id="mongodb">MongoDB</h1>
<p>The database is a container of <strong>collections</strong>. The collections are containers of <strong>documents</strong>.</p>
<p>The documents are <em>schema-less</em> that is they have a dynamic structure that can change between documents in the same collection.</p>
<h2 id="data-types">Data Types</h2>
<table>
<thead>
<tr>
<th>Type</th>
<th>Example Value</th>
<th>Function</th>
</tr>
</thead>
<tbody>
<tr>
<td>Text</td>
<td><code>"Text"</code></td>
<td></td>
</tr>
<tr>
<td>Boolean</td>
<td><code>true</code></td>
<td></td>
</tr>
<tr>
<td>Number</td>
<td><code>42</code></td>
<td></td>
</tr>
<tr>
<td>Objectid</td>
<td><code>"_id": {"$oid": "&lt;id&gt;"}</code></td>
<td><code>ObjectId("&lt;id&gt;")</code></td>
</tr>
<tr>
<td>ISODate</td>
<td><code>"&lt;key&gt;": {"$date": "YYYY-MM-DDThh:mm:ss.sssZ"}</code></td>
<td><code>ISODate("YYYY-MM-DD")</code></td>
</tr>
<tr>
<td>Timestamp</td>
<td></td>
<td><code>Timestamp(11421532)</code></td>
</tr>
<tr>
<td>Embedded Document</td>
<td><code>{"a": {...}}</code></td>
<td></td>
</tr>
<tr>
<td>Embedded Array</td>
<td><code>{"b": [...]}</code></td>
<td></td>
</tr>
</tbody>
</table>
<p>It's mandatory for each document ot have an unique field <code>_id</code>.
MongoDB automatically creates an <code>ObjectId()</code> if it's not provided.</p>
<h2 id="databases-collections-usage">Databases &amp; Collections Usage</h2>
<p>To create a database is sufficient to switch towards a non existing one with <code>use &lt;database&gt;</code> (implicit creation).
The database is not actually created until a document is inserted.</p>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-0-1">1</a></span>
<span class="normal"><a href="#__codelineno-0-2">2</a></span>
<span class="normal"><a href="#__codelineno-0-3">3</a></span>
<span class="normal"><a href="#__codelineno-0-4">4</a></span>
<span class="normal"><a href="#__codelineno-0-5">5</a></span>
<span class="normal"><a href="#__codelineno-0-6">6</a></span>
<span class="normal"><a href="#__codelineno-0-7">7</a></span>
<span class="normal"><a href="#__codelineno-0-8">8</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-0-1" name="__codelineno-0-1"></a>show<span class="w"> </span>dbs<span class="w"> </span><span class="c1"># list all databases</span>
<a id="__codelineno-0-2" name="__codelineno-0-2"></a>use<span class="w"> </span>&lt;database&gt;<span class="w"> </span><span class="c1"># use a particular database</span>
<a id="__codelineno-0-3" name="__codelineno-0-3"></a>show<span class="w"> </span>collections<span class="w"> </span><span class="c1"># list all collection for the current database</span>
<a id="__codelineno-0-4" name="__codelineno-0-4"></a>
<a id="__codelineno-0-5" name="__codelineno-0-5"></a>dbs.dropDatabase<span class="o">()</span><span class="w"> </span><span class="c1"># delete current database</span>
<a id="__codelineno-0-6" name="__codelineno-0-6"></a>
<a id="__codelineno-0-7" name="__codelineno-0-7"></a>db.createCollection<span class="o">(</span>name,<span class="w"> </span><span class="o">{</span>options<span class="o">})</span><span class="w"> </span><span class="c1"># explicit collection creation</span>
<a id="__codelineno-0-8" name="__codelineno-0-8"></a>db.&lt;collection&gt;.insertOne<span class="o">({</span>document<span class="o">})</span><span class="w"> </span><span class="c1"># implicit collection creation</span>
</code></pre></div></td></tr></table></div>
<h2 id="operators-mql-syntax">Operators (MQL Syntax)</h2>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">JSON</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-1-1"> 1</a></span>
<span class="normal"><a href="#__codelineno-1-2"> 2</a></span>
<span class="normal"><a href="#__codelineno-1-3"> 3</a></span>
<span class="normal"><a href="#__codelineno-1-4"> 4</a></span>
<span class="normal"><a href="#__codelineno-1-5"> 5</a></span>
<span class="normal"><a href="#__codelineno-1-6"> 6</a></span>
<span class="normal"><a href="#__codelineno-1-7"> 7</a></span>
<span class="normal"><a href="#__codelineno-1-8"> 8</a></span>
<span class="normal"><a href="#__codelineno-1-9"> 9</a></span>
<span class="normal"><a href="#__codelineno-1-10">10</a></span>
<span class="normal"><a href="#__codelineno-1-11">11</a></span>
<span class="normal"><a href="#__codelineno-1-12">12</a></span>
<span class="normal"><a href="#__codelineno-1-13">13</a></span>
<span class="normal"><a href="#__codelineno-1-14">14</a></span>
<span class="normal"><a href="#__codelineno-1-15">15</a></span>
<span class="normal"><a href="#__codelineno-1-16">16</a></span>
<span class="normal"><a href="#__codelineno-1-17">17</a></span>
<span class="normal"><a href="#__codelineno-1-18">18</a></span>
<span class="normal"><a href="#__codelineno-1-19">19</a></span>
<span class="normal"><a href="#__codelineno-1-20">20</a></span>
<span class="normal"><a href="#__codelineno-1-21">21</a></span>
<span class="normal"><a href="#__codelineno-1-22">22</a></span>
<span class="normal"><a href="#__codelineno-1-23">23</a></span>
<span class="normal"><a href="#__codelineno-1-24">24</a></span>
<span class="normal"><a href="#__codelineno-1-25">25</a></span>
<span class="normal"><a href="#__codelineno-1-26">26</a></span>
<span class="normal"><a href="#__codelineno-1-27">27</a></span>
<span class="normal"><a href="#__codelineno-1-28">28</a></span>
<span class="normal"><a href="#__codelineno-1-29">29</a></span>
<span class="normal"><a href="#__codelineno-1-30">30</a></span>
<span class="normal"><a href="#__codelineno-1-31">31</a></span>
<span class="normal"><a href="#__codelineno-1-32">32</a></span>
<span class="normal"><a href="#__codelineno-1-33">33</a></span>
<span class="normal"><a href="#__codelineno-1-34">34</a></span>
<span class="normal"><a href="#__codelineno-1-35">35</a></span>
<span class="normal"><a href="#__codelineno-1-36">36</a></span>
<span class="normal"><a href="#__codelineno-1-37">37</a></span>
<span class="normal"><a href="#__codelineno-1-38">38</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-1-1" name="__codelineno-1-1"></a><span class="cm">/* --- Update operators --- */</span>
<a id="__codelineno-1-2" name="__codelineno-1-2"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$inc&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;&lt;increment&gt;&quot;</span><span class="p">,</span><span class="w"> </span><span class="err">...</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="c1">// increment value</span>
<a id="__codelineno-1-3" name="__codelineno-1-3"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$set&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;&lt;value&gt;&quot;</span><span class="p">,</span><span class="w"> </span><span class="err">...</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="c1">// set value</span>
<a id="__codelineno-1-4" name="__codelineno-1-4"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$unsetset&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;true&quot;</span><span class="p">,</span><span class="w"> </span><span class="err">...</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="c1">// unset value</span>
<a id="__codelineno-1-5" name="__codelineno-1-5"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$push&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;&lt;value&gt;&quot;</span><span class="p">,</span><span class="w"> </span><span class="err">...</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="c1">// add a value to an array field or turn field into array</span>
<a id="__codelineno-1-6" name="__codelineno-1-6"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$pop&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="err">&lt;</span><span class="mi">-1</span><span class="w"> </span><span class="err">|</span><span class="w"> </span><span class="mi">1</span><span class="err">&gt;</span><span class="p">,</span><span class="w"> </span><span class="err">...</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="c1">// remove first or last array element</span>
<a id="__codelineno-1-7" name="__codelineno-1-7"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$pull&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;&lt;value | condition&gt;&quot;</span><span class="p">,</span><span class="w"> </span><span class="err">...</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="c1">// remove a specific item in the array</span>
<a id="__codelineno-1-8" name="__codelineno-1-8"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$pullAll&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">[</span><span class="w"> </span><span class="s2">&quot;&lt;value1&gt;&quot;</span><span class="p">,</span><span class="w"> </span><span class="s2">&quot;&lt;value2&gt;&quot;</span><span class="w"> </span><span class="err">...</span><span class="w"> </span><span class="p">],</span><span class="w"> </span><span class="err">...</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="c1">// remove all specified values from array</span>
<a id="__codelineno-1-9" name="__codelineno-1-9"></a>
<a id="__codelineno-1-10" name="__codelineno-1-10"></a><span class="cm">/* --- Query Operators --- */</span>
<a id="__codelineno-1-11" name="__codelineno-1-11"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$in&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">[</span><span class="w"> </span><span class="s2">&quot;&lt;value_1&gt;&quot;</span><span class="p">,</span><span class="w"> </span><span class="s2">&quot;&lt;value_2&gt;&quot;</span><span class="p">,</span><span class="w"> </span><span class="err">...</span><span class="p">]</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="c1">// Membership</span>
<a id="__codelineno-1-12" name="__codelineno-1-12"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$nin&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">[</span><span class="w"> </span><span class="s2">&quot;&lt;value_1&gt;&quot;</span><span class="p">,</span><span class="w"> </span><span class="s2">&quot;&lt;value_2&gt;&quot;</span><span class="p">,</span><span class="w"> </span><span class="err">...</span><span class="p">]</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="c1">// Membership</span>
<a id="__codelineno-1-13" name="__codelineno-1-13"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$exists&quot;</span><span class="p">:</span><span class="w"> </span><span class="kc">true</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="c1">// Field Exists</span>
<a id="__codelineno-1-14" name="__codelineno-1-14"></a>
<a id="__codelineno-1-15" name="__codelineno-1-15"></a><span class="cm">/* --- Comparison Operators (DEFAULT: $eq) --- */</span>
<a id="__codelineno-1-16" name="__codelineno-1-16"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$gt&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;&lt;value&gt;&quot;</span><span class="w"> </span><span class="p">}}</span><span class="w"> </span><span class="c1">// &gt;</span>
<a id="__codelineno-1-17" name="__codelineno-1-17"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$gte&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;&lt;value&gt;&quot;</span><span class="w"> </span><span class="p">}}</span><span class="w"> </span><span class="c1">// &gt;=</span>
<a id="__codelineno-1-18" name="__codelineno-1-18"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$lt&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;&lt;value&gt;&quot;</span><span class="w"> </span><span class="p">}}</span><span class="w"> </span><span class="c1">// &lt;</span>
<a id="__codelineno-1-19" name="__codelineno-1-19"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$lte&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;&lt;value&gt;&quot;</span><span class="w"> </span><span class="p">}}</span><span class="w"> </span><span class="c1">// &lt;=</span>
<a id="__codelineno-1-20" name="__codelineno-1-20"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$eq&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;&lt;value&gt;&quot;</span><span class="w"> </span><span class="p">}}</span><span class="w"> </span><span class="c1">// ==</span>
<a id="__codelineno-1-21" name="__codelineno-1-21"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$ne&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;&lt;value&gt;&quot;</span><span class="w"> </span><span class="p">}}</span><span class="w"> </span><span class="c1">// !=</span>
<a id="__codelineno-1-22" name="__codelineno-1-22"></a>
<a id="__codelineno-1-23" name="__codelineno-1-23"></a><span class="cm">/* --- Logic Operators (DEFAULT $and) --- */</span>
<a id="__codelineno-1-24" name="__codelineno-1-24"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$and&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">[</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="s2">&quot;&lt;expression&gt;&quot;</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="err">...</span><span class="p">]</span><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-1-25" name="__codelineno-1-25"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$or&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">[</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="s2">&quot;&lt;expression&gt;&quot;</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="err">...</span><span class="p">]</span><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-1-26" name="__codelineno-1-26"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$nor&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">[</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="s2">&quot;&lt;expression&gt;&quot;</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="err">...</span><span class="p">]</span><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-1-27" name="__codelineno-1-27"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$not&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="s2">&quot;&lt;expression&gt;&quot;</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-1-28" name="__codelineno-1-28"></a>
<a id="__codelineno-1-29" name="__codelineno-1-29"></a><span class="cm">/* --- Array Operators --- */</span>
<a id="__codelineno-1-30" name="__codelineno-1-30"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$all&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">[</span><span class="s2">&quot;value&gt;&quot;</span><span class="p">,</span><span class="w"> </span><span class="s2">&quot;&lt;value&gt;&quot;</span><span class="p">,</span><span class="w"> </span><span class="err">...</span><span class="p">]</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="c1">// field contains all values</span>
<a id="__codelineno-1-31" name="__codelineno-1-31"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$size&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;&lt;value&gt;&quot;</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-1-32" name="__codelineno-1-32"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;array-key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$elemMatch&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;item-key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;&lt;expression&gt;&quot;</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="c1">// elements in array must match an expression</span>
<a id="__codelineno-1-33" name="__codelineno-1-33"></a>
<a id="__codelineno-1-34" name="__codelineno-1-34"></a><span class="cm">/* --- REGEX Operator --- */</span>
<a id="__codelineno-1-35" name="__codelineno-1-35"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$regex&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;/pattern/&quot;</span><span class="p">,</span><span class="w"> </span><span class="nt">&quot;$options&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;&lt;options&gt;&quot;</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-1-36" name="__codelineno-1-36"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$regex&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;pattern&quot;</span><span class="p">,</span><span class="w"> </span><span class="nt">&quot;$options&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;&lt;options&gt;&quot;</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-1-37" name="__codelineno-1-37"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$regex&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;/pattern/&lt;options&gt;&quot;</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-1-38" name="__codelineno-1-38"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;&lt;key&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="s2">&quot;/pattern/&lt;options&gt;&quot;</span><span class="w"> </span><span class="p">}</span>
</code></pre></div></td></tr></table></div>
<h3 id="expressive-query-operator">Expressive Query Operator</h3>
<blockquote>
<p><strong>Note</strong>: <code>$&lt;key&gt;</code> is used to access the value of the field dynamically</p>
</blockquote>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">JSON</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-2-1">1</a></span>
<span class="normal"><a href="#__codelineno-2-2">2</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-2-1" name="__codelineno-2-1"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$expr&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="s2">&quot;&lt;expression&gt;&quot;</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="c1">// aggregation expression, variables, conditional expressions</span>
<a id="__codelineno-2-2" name="__codelineno-2-2"></a><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$expr&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nt">&quot;$&lt;comparison_operator&gt;&quot;</span><span class="p">:</span><span class="w"> </span><span class="p">[</span><span class="w"> </span><span class="s2">&quot;$&lt;key&gt;&quot;</span><span class="p">,</span><span class="w"> </span><span class="s2">&quot;$&lt;key&gt;&quot;</span><span class="w"> </span><span class="p">]</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="c1">// compare field values (operators use aggregation syntax)</span>
</code></pre></div></td></tr></table></div>
<h2 id="mongo-query-language-mql">Mongo Query Language (MQL)</h2>
<h3 id="insertion">Insertion</h3>
<p>It's possible to insert a single document with the command <code>insertOne()</code> or multiple documents with <code>insertMany()</code>.</p>
<p>Insertion results:</p>
<ul>
<li>error -&gt; rollback</li>
<li>success -&gt; entire documents gets saved</li>
</ul>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-3-1"> 1</a></span>
<span class="normal"><a href="#__codelineno-3-2"> 2</a></span>
<span class="normal"><a href="#__codelineno-3-3"> 3</a></span>
<span class="normal"><a href="#__codelineno-3-4"> 4</a></span>
<span class="normal"><a href="#__codelineno-3-5"> 5</a></span>
<span class="normal"><a href="#__codelineno-3-6"> 6</a></span>
<span class="normal"><a href="#__codelineno-3-7"> 7</a></span>
<span class="normal"><a href="#__codelineno-3-8"> 8</a></span>
<span class="normal"><a href="#__codelineno-3-9"> 9</a></span>
<span class="normal"><a href="#__codelineno-3-10">10</a></span>
<span class="normal"><a href="#__codelineno-3-11">11</a></span>
<span class="normal"><a href="#__codelineno-3-12">12</a></span>
<span class="normal"><a href="#__codelineno-3-13">13</a></span>
<span class="normal"><a href="#__codelineno-3-14">14</a></span>
<span class="normal"><a href="#__codelineno-3-15">15</a></span>
<span class="normal"><a href="#__codelineno-3-16">16</a></span>
<span class="normal"><a href="#__codelineno-3-17">17</a></span>
<span class="normal"><a href="#__codelineno-3-18">18</a></span>
<span class="normal"><a href="#__codelineno-3-19">19</a></span>
<span class="normal"><a href="#__codelineno-3-20">20</a></span>
<span class="normal"><a href="#__codelineno-3-21">21</a></span>
<span class="normal"><a href="#__codelineno-3-22">22</a></span>
<span class="normal"><a href="#__codelineno-3-23">23</a></span>
<span class="normal"><a href="#__codelineno-3-24">24</a></span>
<span class="normal"><a href="#__codelineno-3-25">25</a></span>
<span class="normal"><a href="#__codelineno-3-26">26</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-3-1" name="__codelineno-3-1"></a><span class="c1"># explicit collection creation, all options are optional</span>
<a id="__codelineno-3-2" name="__codelineno-3-2"></a>db.createCollection<span class="o">(</span><span class="w"> </span>&lt;name&gt;,
<a id="__codelineno-3-3" name="__codelineno-3-3"></a><span class="w"> </span><span class="o">{</span>
<a id="__codelineno-3-4" name="__codelineno-3-4"></a><span class="w"> </span>capped:<span class="w"> </span>&lt;boolean&gt;,
<a id="__codelineno-3-5" name="__codelineno-3-5"></a><span class="w"> </span>autoIndexId:<span class="w"> </span>&lt;boolean&gt;,
<a id="__codelineno-3-6" name="__codelineno-3-6"></a><span class="w"> </span>size:<span class="w"> </span>&lt;number&gt;,
<a id="__codelineno-3-7" name="__codelineno-3-7"></a><span class="w"> </span>max:<span class="w"> </span>&lt;number&gt;,
<a id="__codelineno-3-8" name="__codelineno-3-8"></a><span class="w"> </span>storageEngine:<span class="w"> </span>&lt;document&gt;,
<a id="__codelineno-3-9" name="__codelineno-3-9"></a><span class="w"> </span>validator:<span class="w"> </span>&lt;document&gt;,
<a id="__codelineno-3-10" name="__codelineno-3-10"></a><span class="w"> </span>validationLevel:<span class="w"> </span>&lt;string&gt;,
<a id="__codelineno-3-11" name="__codelineno-3-11"></a><span class="w"> </span>validationAction:<span class="w"> </span>&lt;string&gt;,
<a id="__codelineno-3-12" name="__codelineno-3-12"></a><span class="w"> </span>indexOptionDefaults:<span class="w"> </span>&lt;document&gt;,
<a id="__codelineno-3-13" name="__codelineno-3-13"></a><span class="w"> </span>viewOn:<span class="w"> </span>&lt;string&gt;,
<a id="__codelineno-3-14" name="__codelineno-3-14"></a><span class="w"> </span>pipeline:<span class="w"> </span>&lt;pipeline&gt;,
<a id="__codelineno-3-15" name="__codelineno-3-15"></a><span class="w"> </span>collation:<span class="w"> </span>&lt;document&gt;,
<a id="__codelineno-3-16" name="__codelineno-3-16"></a><span class="w"> </span>writeConcern:<span class="w"> </span>&lt;document&gt;
<a id="__codelineno-3-17" name="__codelineno-3-17"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-3-18" name="__codelineno-3-18"></a><span class="o">)</span>
<a id="__codelineno-3-19" name="__codelineno-3-19"></a>
<a id="__codelineno-3-20" name="__codelineno-3-20"></a>db.createCollection<span class="o">(</span><span class="s2">&quot;name&quot;</span>,<span class="w"> </span><span class="o">{</span><span class="w"> </span>capped:<span class="w"> </span>true,<span class="w"> </span>size:<span class="w"> </span>max_bytes,<span class="w"> </span>max:<span class="w"> </span>max_docs_num<span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">)</span><span class="w"> </span><span class="c1"># creation of a capped collection</span>
<a id="__codelineno-3-21" name="__codelineno-3-21"></a><span class="c1"># SIZE: int - will be rounded to a multiple of 256</span>
<a id="__codelineno-3-22" name="__codelineno-3-22"></a>
<a id="__codelineno-3-23" name="__codelineno-3-23"></a><span class="c1"># implicit creation at doc insertion</span>
<a id="__codelineno-3-24" name="__codelineno-3-24"></a>db.&lt;collection&gt;.insertOne<span class="o">({</span><span class="w"> </span>document<span class="w"> </span><span class="o">}</span>,<span class="w"> </span>options<span class="o">)</span><span class="w"> </span><span class="c1"># insert a document in a collection</span>
<a id="__codelineno-3-25" name="__codelineno-3-25"></a>db.&lt;collection&gt;.insertMany<span class="o">([</span><span class="w"> </span><span class="o">{</span><span class="w"> </span>document<span class="w"> </span><span class="o">}</span>,<span class="w"> </span><span class="o">{</span><span class="w"> </span>document<span class="w"> </span><span class="o">}</span>,<span class="w"> </span>...<span class="w"> </span><span class="o">]</span>,<span class="w"> </span>options<span class="o">)</span><span class="w"> </span><span class="c1"># insert multiple docs</span>
<a id="__codelineno-3-26" name="__codelineno-3-26"></a>db.&lt;collection&gt;.insertMany<span class="o">([</span><span class="w"> </span><span class="o">{</span><span class="w"> </span>document<span class="w"> </span><span class="o">}</span>,<span class="w"> </span><span class="o">{</span><span class="w"> </span>document<span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">]</span><span class="w"> </span>,<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;ordered&quot;</span>:<span class="w"> </span><span class="nb">false</span><span class="w"> </span><span class="o">})</span><span class="w"> </span><span class="c1"># allow the unordered insertion, only documents that cause errors wont be inserted</span>
</code></pre></div></td></tr></table></div>
<blockquote>
<p><strong>Note</strong>: If <code>insertMany()</code> fails the already inserted documents are not rolled back but all the successive ones (even the correct ones) will not be inserted.</p>
</blockquote>
<h3 id="querying">Querying</h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-4-1"> 1</a></span>
<span class="normal"><a href="#__codelineno-4-2"> 2</a></span>
<span class="normal"><a href="#__codelineno-4-3"> 3</a></span>
<span class="normal"><a href="#__codelineno-4-4"> 4</a></span>
<span class="normal"><a href="#__codelineno-4-5"> 5</a></span>
<span class="normal"><a href="#__codelineno-4-6"> 6</a></span>
<span class="normal"><a href="#__codelineno-4-7"> 7</a></span>
<span class="normal"><a href="#__codelineno-4-8"> 8</a></span>
<span class="normal"><a href="#__codelineno-4-9"> 9</a></span>
<span class="normal"><a href="#__codelineno-4-10">10</a></span>
<span class="normal"><a href="#__codelineno-4-11">11</a></span>
<span class="normal"><a href="#__codelineno-4-12">12</a></span>
<span class="normal"><a href="#__codelineno-4-13">13</a></span>
<span class="normal"><a href="#__codelineno-4-14">14</a></span>
<span class="normal"><a href="#__codelineno-4-15">15</a></span>
<span class="normal"><a href="#__codelineno-4-16">16</a></span>
<span class="normal"><a href="#__codelineno-4-17">17</a></span>
<span class="normal"><a href="#__codelineno-4-18">18</a></span>
<span class="normal"><a href="#__codelineno-4-19">19</a></span>
<span class="normal"><a href="#__codelineno-4-20">20</a></span>
<span class="normal"><a href="#__codelineno-4-21">21</a></span>
<span class="normal"><a href="#__codelineno-4-22">22</a></span>
<span class="normal"><a href="#__codelineno-4-23">23</a></span>
<span class="normal"><a href="#__codelineno-4-24">24</a></span>
<span class="normal"><a href="#__codelineno-4-25">25</a></span>
<span class="normal"><a href="#__codelineno-4-26">26</a></span>
<span class="normal"><a href="#__codelineno-4-27">27</a></span>
<span class="normal"><a href="#__codelineno-4-28">28</a></span>
<span class="normal"><a href="#__codelineno-4-29">29</a></span>
<span class="normal"><a href="#__codelineno-4-30">30</a></span>
<span class="normal"><a href="#__codelineno-4-31">31</a></span>
<span class="normal"><a href="#__codelineno-4-32">32</a></span>
<span class="normal"><a href="#__codelineno-4-33">33</a></span>
<span class="normal"><a href="#__codelineno-4-34">34</a></span>
<span class="normal"><a href="#__codelineno-4-35">35</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-4-1" name="__codelineno-4-1"></a>db.&lt;collection&gt;.findOne<span class="o">()</span><span class="w"> </span><span class="c1"># find only one document</span>
<a id="__codelineno-4-2" name="__codelineno-4-2"></a>db.&lt;collection&gt;.find<span class="o">(</span>filter<span class="o">)</span><span class="w"> </span><span class="c1"># show selected documents</span>
<a id="__codelineno-4-3" name="__codelineno-4-3"></a>db.&lt;collection&gt;.find<span class="o">()</span>.pretty<span class="o">()</span><span class="w"> </span><span class="c1"># show documents formatted</span>
<a id="__codelineno-4-4" name="__codelineno-4-4"></a>db.&lt;collection&gt;.find<span class="o">()</span>.limit<span class="o">(</span>n<span class="o">)</span><span class="w"> </span><span class="c1"># show n documents</span>
<a id="__codelineno-4-5" name="__codelineno-4-5"></a>db.&lt;collection&gt;.find<span class="o">()</span>.limit<span class="o">(</span>n<span class="o">)</span>.skip<span class="o">(</span>k<span class="o">)</span><span class="w"> </span><span class="c1"># show n documents skipping k docs</span>
<a id="__codelineno-4-6" name="__codelineno-4-6"></a>db.&lt;collection&gt;.find<span class="o">()</span>.count<span class="o">()</span><span class="w"> </span><span class="c1"># number of found docs</span>
<a id="__codelineno-4-7" name="__codelineno-4-7"></a>db.&lt;collection&gt;.find<span class="o">()</span>.sort<span class="o">({</span><span class="w"> </span><span class="s2">&quot;&lt;key-1&gt;&quot;</span>:<span class="w"> </span><span class="m">1</span>,<span class="w"> </span>...<span class="w"> </span>,<span class="w"> </span><span class="s2">&quot;&lt;key-n&gt;&quot;</span>:<span class="w"> </span>-1<span class="w"> </span><span class="o">})</span><span class="w"> </span><span class="c1"># show documents sorted by specified keys in ascending (1) or descending (-1) order</span>
<a id="__codelineno-4-8" name="__codelineno-4-8"></a>
<a id="__codelineno-4-9" name="__codelineno-4-9"></a><span class="c1"># projection</span>
<a id="__codelineno-4-10" name="__codelineno-4-10"></a>db.&lt;collection&gt;.find<span class="o">(</span>filter,<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;&lt;key&gt;&quot;</span>:<span class="w"> </span><span class="m">1</span><span class="w"> </span><span class="o">})</span><span class="w"> </span><span class="c1"># show selected values form documents (1 or true =&gt; show, 0 or false =&gt; don&#39;t show, cant mix 0 and 1)</span>
<a id="__codelineno-4-11" name="__codelineno-4-11"></a>db.&lt;collection&gt;.find<span class="o">(</span>filter,<span class="w"> </span><span class="o">{</span><span class="w"> </span>_id:<span class="w"> </span><span class="m">0</span>,<span class="w"> </span><span class="s2">&quot;&lt;key&gt;&quot;</span>:<span class="w"> </span><span class="m">1</span><span class="w"> </span><span class="o">})</span><span class="w"> </span><span class="c1"># only _id can be set to 0 with other keys at 1</span>
<a id="__codelineno-4-12" name="__codelineno-4-12"></a>db.&lt;collection&gt;.find<span class="o">(</span>filter,<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;&lt;array-key&gt;&quot;</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$elemMatch</span><span class="s2">&quot;</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;&lt;item-key&gt;&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;expression&gt;&quot;</span><span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">})</span><span class="w"> </span><span class="c1"># project only elements matching the expression</span>
<a id="__codelineno-4-13" name="__codelineno-4-13"></a>
<a id="__codelineno-4-14" name="__codelineno-4-14"></a><span class="c1"># sub documents &amp; arrays</span>
<a id="__codelineno-4-15" name="__codelineno-4-15"></a>db.&lt;collection&gt;.find<span class="o">({</span><span class="w"> </span><span class="s2">&quot;&lt;key&gt;.&lt;sub-key&gt;.&lt;sub-key&gt;&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;expression&gt;&quot;</span><span class="w"> </span><span class="o">})</span>
<a id="__codelineno-4-16" name="__codelineno-4-16"></a>db.&lt;collection&gt;.find<span class="o">({</span><span class="w"> </span><span class="s2">&quot;&lt;array-key&gt;.&lt;index&gt;.&lt;sub-key&gt;&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;expression&gt;&quot;</span><span class="w"> </span><span class="o">})</span>
<a id="__codelineno-4-17" name="__codelineno-4-17"></a>
<a id="__codelineno-4-18" name="__codelineno-4-18"></a><span class="c1"># GeoJSON - https://docs.mongodb.com/manual/reference/operator/query/near/index.html</span>
<a id="__codelineno-4-19" name="__codelineno-4-19"></a>db.&lt;collection&gt;.find<span class="o">(</span>
<a id="__codelineno-4-20" name="__codelineno-4-20"></a><span class="w"> </span><span class="o">{</span>
<a id="__codelineno-4-21" name="__codelineno-4-21"></a><span class="w"> </span>&lt;location<span class="w"> </span>field&gt;:<span class="w"> </span><span class="o">{</span>
<a id="__codelineno-4-22" name="__codelineno-4-22"></a><span class="w"> </span><span class="nv">$near</span>:<span class="w"> </span><span class="o">{</span>
<a id="__codelineno-4-23" name="__codelineno-4-23"></a><span class="w"> </span><span class="nv">$geometry</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span>type:<span class="w"> </span><span class="s2">&quot;Point&quot;</span>,<span class="w"> </span>coordinates:<span class="w"> </span><span class="o">[</span><span class="w"> </span>&lt;longitude&gt;<span class="w"> </span>,<span class="w"> </span>&lt;latitude&gt;<span class="w"> </span><span class="o">]</span><span class="w"> </span><span class="o">}</span>,
<a id="__codelineno-4-24" name="__codelineno-4-24"></a><span class="w"> </span><span class="nv">$maxDistance</span>:<span class="w"> </span>&lt;distance<span class="w"> </span><span class="k">in</span><span class="w"> </span>meters&gt;,
<a id="__codelineno-4-25" name="__codelineno-4-25"></a><span class="w"> </span><span class="nv">$minDistance</span>:<span class="w"> </span>&lt;distance<span class="w"> </span><span class="k">in</span><span class="w"> </span>meters&gt;
<a id="__codelineno-4-26" name="__codelineno-4-26"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-4-27" name="__codelineno-4-27"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-4-28" name="__codelineno-4-28"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-4-29" name="__codelineno-4-29"></a><span class="o">)</span>
<a id="__codelineno-4-30" name="__codelineno-4-30"></a>
<a id="__codelineno-4-31" name="__codelineno-4-31"></a>db.&lt;collection&gt;.find<span class="o">()</span>.hint<span class="o">(</span><span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;&lt;key&gt;&quot;</span>:<span class="w"> </span><span class="m">1</span><span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">)</span><span class="w"> </span><span class="c1"># specify the index</span>
<a id="__codelineno-4-32" name="__codelineno-4-32"></a>db.&lt;collection&gt;.find<span class="o">()</span>.hint<span class="o">(</span><span class="w"> </span><span class="s2">&quot;index-name&quot;</span><span class="w"> </span><span class="o">)</span><span class="w"> </span><span class="c1"># specify the index using the index name</span>
<a id="__codelineno-4-33" name="__codelineno-4-33"></a>
<a id="__codelineno-4-34" name="__codelineno-4-34"></a>db.&lt;collection&gt;.find<span class="o">()</span>.hint<span class="o">(</span><span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="nv">$natural</span><span class="w"> </span>:<span class="w"> </span><span class="m">1</span><span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">)</span><span class="w"> </span><span class="c1"># force the query to perform a forwards collection scan</span>
<a id="__codelineno-4-35" name="__codelineno-4-35"></a>db.&lt;collection&gt;.find<span class="o">()</span>.hint<span class="o">(</span><span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="nv">$natural</span><span class="w"> </span>:<span class="w"> </span>-1<span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">)</span><span class="w"> </span><span class="c1"># force the query to perform a reverse collection scan</span>
</code></pre></div></td></tr></table></div>
<blockquote>
<p><strong>Note</strong>: <code>{ &lt;key&gt;: &lt;value&gt; }</code> in case of a field array will match if the array <em>contains</em> the value</p>
</blockquote>
<h3 id="updating">Updating</h3>
<p><a href="https://docs.mongodb.com/manual/reference/operator/update/" title="Update Operators Documentation">Update Operators</a></p>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-5-1">1</a></span>
<span class="normal"><a href="#__codelineno-5-2">2</a></span>
<span class="normal"><a href="#__codelineno-5-3">3</a></span>
<span class="normal"><a href="#__codelineno-5-4">4</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-5-1" name="__codelineno-5-1"></a>db.&lt;collection&gt;.replaceOne<span class="o">(</span>filter,<span class="w"> </span>update,<span class="w"> </span>options<span class="o">)</span>
<a id="__codelineno-5-2" name="__codelineno-5-2"></a>db.&lt;collection&gt;.updateOne<span class="o">(</span>filter,<span class="w"> </span>update,<span class="w"> </span><span class="o">{</span>upsert:<span class="w"> </span>true<span class="o">})</span><span class="w"> </span><span class="c1"># modify document if existing, insert otherwise</span>
<a id="__codelineno-5-3" name="__codelineno-5-3"></a>
<a id="__codelineno-5-4" name="__codelineno-5-4"></a>db.&lt;collection&gt;.updateOne<span class="o">(</span>filter,<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$push</span><span class="s2">&quot;</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span>...<span class="w"> </span><span class="o">}</span>,<span class="w"> </span><span class="s2">&quot;</span><span class="nv">$set</span><span class="s2">&quot;</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span>...<span class="w"> </span><span class="o">}</span>,<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$inc</span><span class="s2">&quot;</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span>...<span class="w"> </span><span class="o">}</span>,<span class="w"> </span>...<span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">})</span>
</code></pre></div></td></tr></table></div>
<h3 id="deletion">Deletion</h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-6-1">1</a></span>
<span class="normal"><a href="#__codelineno-6-2">2</a></span>
<span class="normal"><a href="#__codelineno-6-3">3</a></span>
<span class="normal"><a href="#__codelineno-6-4">4</a></span>
<span class="normal"><a href="#__codelineno-6-5">5</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-6-1" name="__codelineno-6-1"></a>db.&lt;collection&gt;.deleteOne<span class="o">(</span>filter,<span class="w"> </span>options<span class="o">)</span>
<a id="__codelineno-6-2" name="__codelineno-6-2"></a>db.&lt;collection&gt;.deleteMany<span class="o">(</span>filter,<span class="w"> </span>options<span class="o">)</span>
<a id="__codelineno-6-3" name="__codelineno-6-3"></a>
<a id="__codelineno-6-4" name="__codelineno-6-4"></a>db.&lt;collection&gt;.drop<span class="o">()</span><span class="w"> </span><span class="c1"># delete whole collection</span>
<a id="__codelineno-6-5" name="__codelineno-6-5"></a>db.dropDatabase<span class="o">()</span><span class="w"> </span><span class="c1"># delete entire database</span>
</code></pre></div></td></tr></table></div>
<hr />
<h2 id="mongodb-database-tools">MongoDB Database Tools</h2>
<h3 id="mongoimport"><a href="https://docs.mongodb.com/database-tools/mongoimport/">Mongoimport</a></h3>
<p>Utility to import all docs into a specified collection.<br />
If the collection already exists <code>--drop</code> deletes it before reuploading it.
<strong>WARNING</strong>: CSV separators must be commas (<code>,</code>)</p>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-7-1"> 1</a></span>
<span class="normal"><a href="#__codelineno-7-2"> 2</a></span>
<span class="normal"><a href="#__codelineno-7-3"> 3</a></span>
<span class="normal"><a href="#__codelineno-7-4"> 4</a></span>
<span class="normal"><a href="#__codelineno-7-5"> 5</a></span>
<span class="normal"><a href="#__codelineno-7-6"> 6</a></span>
<span class="normal"><a href="#__codelineno-7-7"> 7</a></span>
<span class="normal"><a href="#__codelineno-7-8"> 8</a></span>
<span class="normal"><a href="#__codelineno-7-9"> 9</a></span>
<span class="normal"><a href="#__codelineno-7-10">10</a></span>
<span class="normal"><a href="#__codelineno-7-11">11</a></span>
<span class="normal"><a href="#__codelineno-7-12">12</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-7-1" name="__codelineno-7-1"></a>mongoimport<span class="w"> </span>&lt;options&gt;<span class="w"> </span>&lt;connection-string&gt;<span class="w"> </span>&lt;file&gt;
<a id="__codelineno-7-2" name="__codelineno-7-2"></a>
<a id="__codelineno-7-3" name="__codelineno-7-3"></a>--uri<span class="o">=</span>&lt;connectionString&gt;
<a id="__codelineno-7-4" name="__codelineno-7-4"></a>--host<span class="o">=</span>&lt;hostname&gt;&lt;:port&gt;,<span class="w"> </span>-h<span class="o">=</span>&lt;hostname&gt;&lt;:port&gt;
<a id="__codelineno-7-5" name="__codelineno-7-5"></a>--username<span class="o">=</span>&lt;username&gt;,<span class="w"> </span>-u<span class="o">=</span>&lt;username&gt;
<a id="__codelineno-7-6" name="__codelineno-7-6"></a>--password<span class="o">=</span>&lt;password&gt;,<span class="w"> </span>-p<span class="o">=</span>&lt;password&gt;
<a id="__codelineno-7-7" name="__codelineno-7-7"></a>--collection<span class="o">=</span>&lt;collection&gt;,<span class="w"> </span>-c<span class="o">=</span>&lt;collection&gt;<span class="w"> </span><span class="c1"># Specifies the collection to import.</span>
<a id="__codelineno-7-8" name="__codelineno-7-8"></a>--ssl<span class="w"> </span><span class="c1"># Enables connection to a mongod or mongos that has TLS/SSL support enabled.</span>
<a id="__codelineno-7-9" name="__codelineno-7-9"></a>--type<span class="w"> </span>&lt;json<span class="p">|</span>csv<span class="p">|</span>tsv&gt;<span class="w"> </span><span class="c1"># Specifies the file type to import. DEFAULT: json</span>
<a id="__codelineno-7-10" name="__codelineno-7-10"></a>--drop<span class="w"> </span><span class="c1"># drops the collection before importing the data from the input.</span>
<a id="__codelineno-7-11" name="__codelineno-7-11"></a>--headerline<span class="w"> </span><span class="c1"># if file is CSV and first line is header</span>
<a id="__codelineno-7-12" name="__codelineno-7-12"></a>--jsonarray<span class="w"> </span><span class="c1"># Accepts the import of data expressed with multiple MongoDB documents within a single json array. MAX 16 MB</span>
</code></pre></div></td></tr></table></div>
<h3 id="mongoexport"><a href="https://docs.mongodb.com/database-tools/mongoexport/">Mongoexport</a></h3>
<p>Utility to export documents into a specified file.</p>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-8-1"> 1</a></span>
<span class="normal"><a href="#__codelineno-8-2"> 2</a></span>
<span class="normal"><a href="#__codelineno-8-3"> 3</a></span>
<span class="normal"><a href="#__codelineno-8-4"> 4</a></span>
<span class="normal"><a href="#__codelineno-8-5"> 5</a></span>
<span class="normal"><a href="#__codelineno-8-6"> 6</a></span>
<span class="normal"><a href="#__codelineno-8-7"> 7</a></span>
<span class="normal"><a href="#__codelineno-8-8"> 8</a></span>
<span class="normal"><a href="#__codelineno-8-9"> 9</a></span>
<span class="normal"><a href="#__codelineno-8-10">10</a></span>
<span class="normal"><a href="#__codelineno-8-11">11</a></span>
<span class="normal"><a href="#__codelineno-8-12">12</a></span>
<span class="normal"><a href="#__codelineno-8-13">13</a></span>
<span class="normal"><a href="#__codelineno-8-14">14</a></span>
<span class="normal"><a href="#__codelineno-8-15">15</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-8-1" name="__codelineno-8-1"></a>mongoexport<span class="w"> </span>--collection<span class="o">=</span>&lt;collection&gt;<span class="w"> </span>&lt;options&gt;<span class="w"> </span>&lt;connection-string&gt;
<a id="__codelineno-8-2" name="__codelineno-8-2"></a>
<a id="__codelineno-8-3" name="__codelineno-8-3"></a>--uri<span class="o">=</span>&lt;connectionString&gt;
<a id="__codelineno-8-4" name="__codelineno-8-4"></a>--host<span class="o">=</span>&lt;hostname&gt;&lt;:port&gt;,<span class="w"> </span>-h<span class="o">=</span>&lt;hostname&gt;&lt;:port&gt;
<a id="__codelineno-8-5" name="__codelineno-8-5"></a>--username<span class="o">=</span>&lt;username&gt;,<span class="w"> </span>-u<span class="o">=</span>&lt;username&gt;
<a id="__codelineno-8-6" name="__codelineno-8-6"></a>--password<span class="o">=</span>&lt;password&gt;,<span class="w"> </span>-p<span class="o">=</span>&lt;password&gt;
<a id="__codelineno-8-7" name="__codelineno-8-7"></a>--db<span class="o">=</span>&lt;database&gt;,<span class="w"> </span>-d<span class="o">=</span>&lt;database&gt;
<a id="__codelineno-8-8" name="__codelineno-8-8"></a>--collection<span class="o">=</span>&lt;collection&gt;,<span class="w"> </span>-c<span class="o">=</span>&lt;collection&gt;
<a id="__codelineno-8-9" name="__codelineno-8-9"></a>--type<span class="o">=</span>&lt;json<span class="p">|</span>csv&gt;
<a id="__codelineno-8-10" name="__codelineno-8-10"></a>--out<span class="o">=</span>&lt;file&gt;,<span class="w"> </span>-o<span class="o">=</span>&lt;file&gt;<span class="w"> </span><span class="c1">#Specifies a file to write the export to. DEFAULT: stdout</span>
<a id="__codelineno-8-11" name="__codelineno-8-11"></a>--jsonArray<span class="w"> </span><span class="c1"># Write the entire contents of the export as a single json array.</span>
<a id="__codelineno-8-12" name="__codelineno-8-12"></a>--pretty<span class="w"> </span><span class="c1"># Outputs documents in a pretty-printed format JSON.</span>
<a id="__codelineno-8-13" name="__codelineno-8-13"></a>--skip<span class="o">=</span>&lt;number&gt;
<a id="__codelineno-8-14" name="__codelineno-8-14"></a>--limit<span class="o">=</span>&lt;number&gt;<span class="w"> </span><span class="c1"># Specifies a maximum number of documents to include in the export</span>
<a id="__codelineno-8-15" name="__codelineno-8-15"></a>--sort<span class="o">=</span>&lt;JSON&gt;<span class="w"> </span><span class="c1"># Specifies an ordering for exported results</span>
</code></pre></div></td></tr></table></div>
<h3 id="mongodump-mongorestore"><a href="https://docs.mongodb.com/database-tools/mongodump/">Mongodump</a> &amp; <a href="https://docs.mongodb.com/database-tools/mongorestore/">Mongorestore</a></h3>
<p><code>mongodump</code> exports the content of a running server into <code>.bson</code> files.</p>
<p><code>mongorestore</code> Restore backups generated with <code>mongodump</code> to a running server.</p>
<hr />
<h2 id="indexes"><a href="https://docs.mongodb.com/manual/indexes/" title="Index Documentation">Indexes</a></h2>
<p>Indexes support the efficient execution of queries in MongoDB.</p>
<p>Without indexes, MongoDB must perform a <em>collection scan</em> (<em>COLLSCAN</em>): scan every document in a collection, to select those documents that match the query statement.<br />
If an appropriate index exists for a query, MongoDB can use the index to limit the number of documents it must inspect (<em>IXSCAN</em>).</p>
<p>Indexes are special data structures that store a small portion of the collection's data set in an easy to traverse form. The index stores the value of a specific field or set of fields, ordered by the value of the field. The ordering of the index entries supports efficient equality matches and range-based query operations. In addition, MongoDB can return sorted results by using the ordering in the index.</p>
<p>Indexes <em>slow down writing operations</em> since the index must be updated at every writing.</p>
<p><img alt="IXSCAN" src="../img/mongodb_ixscan.avif" title=".find() using an index" /></p>
<h3 id="index-types"><a href="https://docs.mongodb.com/manual/indexes/#index-types">Index Types</a></h3>
<ul>
<li><strong>Normal</strong>: Fields sorted by name</li>
<li><strong>Compound</strong>: Multiple Fields sorted by name</li>
<li><strong>Multikey</strong>: values of sorted arrays</li>
<li><strong>Text</strong>: Ordered text fragments</li>
<li><strong>Geospatial</strong>: ordered geodata</li>
</ul>
<p><strong>Sparse</strong> indexes only contain entries for documents that have the indexed field, even if the index field contains a null value. The index skips over any document that is missing the indexed field.</p>
<h3 id="diagnosis-and-query-planning">Diagnosis and query planning</h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-9-1">1</a></span>
<span class="normal"><a href="#__codelineno-9-2">2</a></span>
<span class="normal"><a href="#__codelineno-9-3">3</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-9-1" name="__codelineno-9-1"></a>db.&lt;collection&gt;.find<span class="o">({</span>...<span class="o">})</span>.explain<span class="o">()</span><span class="w"> </span><span class="c1"># explain won&#39;t accept other functions</span>
<a id="__codelineno-9-2" name="__codelineno-9-2"></a>db.explain<span class="o">()</span>.&lt;collection&gt;.find<span class="o">({</span>...<span class="o">})</span><span class="w"> </span><span class="c1"># can accept other functions</span>
<a id="__codelineno-9-3" name="__codelineno-9-3"></a>db.explain<span class="o">(</span><span class="s2">&quot;executionStats&quot;</span><span class="o">)</span>.&lt;collection&gt;.find<span class="o">({</span>...<span class="o">})</span><span class="w"> </span><span class="c1"># more info</span>
</code></pre></div></td></tr></table></div>
<h3 id="index-creation">Index Creation</h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-10-1"> 1</a></span>
<span class="normal"><a href="#__codelineno-10-2"> 2</a></span>
<span class="normal"><a href="#__codelineno-10-3"> 3</a></span>
<span class="normal"><a href="#__codelineno-10-4"> 4</a></span>
<span class="normal"><a href="#__codelineno-10-5"> 5</a></span>
<span class="normal"><a href="#__codelineno-10-6"> 6</a></span>
<span class="normal"><a href="#__codelineno-10-7"> 7</a></span>
<span class="normal"><a href="#__codelineno-10-8"> 8</a></span>
<span class="normal"><a href="#__codelineno-10-9"> 9</a></span>
<span class="normal"><a href="#__codelineno-10-10">10</a></span>
<span class="normal"><a href="#__codelineno-10-11">11</a></span>
<span class="normal"><a href="#__codelineno-10-12">12</a></span>
<span class="normal"><a href="#__codelineno-10-13">13</a></span>
<span class="normal"><a href="#__codelineno-10-14">14</a></span>
<span class="normal"><a href="#__codelineno-10-15">15</a></span>
<span class="normal"><a href="#__codelineno-10-16">16</a></span>
<span class="normal"><a href="#__codelineno-10-17">17</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-10-1" name="__codelineno-10-1"></a>db.&lt;collection&gt;.createIndex<span class="o">(</span><span class="w"> </span>&lt;key<span class="w"> </span>and<span class="w"> </span>index<span class="w"> </span><span class="nb">type</span><span class="w"> </span>specification&gt;,<span class="w"> </span>&lt;options&gt;<span class="w"> </span><span class="o">)</span>
<a id="__codelineno-10-2" name="__codelineno-10-2"></a>
<a id="__codelineno-10-3" name="__codelineno-10-3"></a>db.&lt;collection&gt;.createIndex<span class="o">(</span><span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;&lt;key&gt;&quot;</span>:<span class="w"> </span>&lt;type&gt;,<span class="w"> </span><span class="s2">&quot;&lt;key&gt;&quot;</span>:<span class="w"> </span>&lt;type&gt;,<span class="w"> </span>...<span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">)</span><span class="w"> </span><span class="c1"># normal, compound or multikey (field is array) index</span>
<a id="__codelineno-10-4" name="__codelineno-10-4"></a>db.&lt;collection&gt;.createIndex<span class="o">(</span><span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;&lt;key&gt;&quot;</span>:<span class="w"> </span><span class="s2">&quot;text&quot;</span><span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">)</span><span class="w"> </span><span class="c1"># text index</span>
<a id="__codelineno-10-5" name="__codelineno-10-5"></a>db.&lt;collection&gt;.createIndex<span class="o">(</span><span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;&lt;key&gt;&quot;</span>:<span class="w"> </span>2dsphere<span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">)</span><span class="w"> </span><span class="c1"># geospatial 2dsphere index</span>
<a id="__codelineno-10-6" name="__codelineno-10-6"></a>
<a id="__codelineno-10-7" name="__codelineno-10-7"></a><span class="c1"># sparse index</span>
<a id="__codelineno-10-8" name="__codelineno-10-8"></a>db.&lt;collection&gt;.createIndex<span class="o">(</span>
<a id="__codelineno-10-9" name="__codelineno-10-9"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;&lt;key&gt;&quot;</span>:<span class="w"> </span>&lt;type&gt;,<span class="w"> </span><span class="s2">&quot;&lt;key&gt;&quot;</span>:<span class="w"> </span>&lt;type&gt;,<span class="w"> </span>...<span class="w"> </span><span class="o">}</span>,
<a id="__codelineno-10-10" name="__codelineno-10-10"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span>sparse:<span class="w"> </span><span class="nb">true</span><span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="c1"># sparse option</span>
<a id="__codelineno-10-11" name="__codelineno-10-11"></a><span class="o">)</span>
<a id="__codelineno-10-12" name="__codelineno-10-12"></a>
<a id="__codelineno-10-13" name="__codelineno-10-13"></a><span class="c1"># custom name</span>
<a id="__codelineno-10-14" name="__codelineno-10-14"></a>db.&lt;collection&gt;.createIndex<span class="o">(</span>
<a id="__codelineno-10-15" name="__codelineno-10-15"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span>&lt;key<span class="w"> </span>and<span class="w"> </span>index<span class="w"> </span><span class="nb">type</span><span class="w"> </span>specification&gt;,<span class="w"> </span><span class="o">}</span>,
<a id="__codelineno-10-16" name="__codelineno-10-16"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span>name:<span class="w"> </span><span class="s2">&quot;index-name&quot;</span><span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="c1"># name option</span>
<a id="__codelineno-10-17" name="__codelineno-10-17"></a><span class="o">)</span>
</code></pre></div></td></tr></table></div>
<h3 id="index-management"><a href="https://docs.mongodb.com/manual/tutorial/manage-indexes/">Index Management</a></h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-11-1"> 1</a></span>
<span class="normal"><a href="#__codelineno-11-2"> 2</a></span>
<span class="normal"><a href="#__codelineno-11-3"> 3</a></span>
<span class="normal"><a href="#__codelineno-11-4"> 4</a></span>
<span class="normal"><a href="#__codelineno-11-5"> 5</a></span>
<span class="normal"><a href="#__codelineno-11-6"> 6</a></span>
<span class="normal"><a href="#__codelineno-11-7"> 7</a></span>
<span class="normal"><a href="#__codelineno-11-8"> 8</a></span>
<span class="normal"><a href="#__codelineno-11-9"> 9</a></span>
<span class="normal"><a href="#__codelineno-11-10">10</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-11-1" name="__codelineno-11-1"></a><span class="c1"># view all db indexes</span>
<a id="__codelineno-11-2" name="__codelineno-11-2"></a>db.getCollectionNames<span class="o">()</span>.forEach<span class="o">(</span><span class="k">function</span><span class="o">(</span>collection<span class="o">)</span><span class="w"> </span><span class="o">{</span>
<a id="__codelineno-11-3" name="__codelineno-11-3"></a><span class="w"> </span><span class="nv">indexes</span><span class="w"> </span><span class="o">=</span><span class="w"> </span>db<span class="o">[</span>collection<span class="o">]</span>.getIndexes<span class="o">()</span><span class="p">;</span>
<a id="__codelineno-11-4" name="__codelineno-11-4"></a><span class="w"> </span>print<span class="o">(</span><span class="s2">&quot;Indexes for &quot;</span><span class="w"> </span>+<span class="w"> </span>collection<span class="w"> </span>+<span class="w"> </span><span class="s2">&quot;:&quot;</span><span class="o">)</span><span class="p">;</span>
<a id="__codelineno-11-5" name="__codelineno-11-5"></a><span class="w"> </span>printjson<span class="o">(</span>indexes<span class="o">)</span><span class="p">;</span>
<a id="__codelineno-11-6" name="__codelineno-11-6"></a><span class="o">})</span><span class="p">;</span>
<a id="__codelineno-11-7" name="__codelineno-11-7"></a>db.&lt;collection&gt;.getIndexes<span class="o">()</span><span class="w"> </span><span class="c1"># view collection&#39;s index</span>
<a id="__codelineno-11-8" name="__codelineno-11-8"></a>
<a id="__codelineno-11-9" name="__codelineno-11-9"></a>db.&lt;collection&gt;.dropIndexes<span class="o">()</span><span class="w"> </span><span class="c1"># drop all indexes</span>
<a id="__codelineno-11-10" name="__codelineno-11-10"></a>db.&lt;collection&gt;.dropIndex<span class="o">(</span><span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;index-name&quot;</span>:<span class="w"> </span><span class="m">1</span><span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">)</span><span class="w"> </span><span class="c1"># drop a specific index</span>
</code></pre></div></td></tr></table></div>
<hr />
<h2 id="cluster-administration">Cluster Administration</h2>
<h3 id="mongod"><code>mongod</code></h3>
<p><code>mongod</code> is the main deamon process for MongoDB. It's the core process of the database,
handling connections, requests and persisting the data.</p>
<p><code>mongod</code> default configuration:</p>
<ul>
<li>port: <code>27017</code></li>
<li>dbpath: <code>/data/db</code></li>
<li>bind_ip: <code>localhost</code></li>
<li>auth: disabled</li>
</ul>
<p><a href="https://www.mongodb.com/docs/manual/reference/configuration-options" title="`mongod` config file docs"><code>mongod</code> config file</a><br />
<a href="https://www.mongodb.com/docs/manual/reference/program/mongod/#options" title="`mongod` command line options docs"><code>mongod</code> command line options</a></p>
<h3 id="basic-shell-helpers">Basic Shell Helpers</h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-12-1"> 1</a></span>
<span class="normal"><a href="#__codelineno-12-2"> 2</a></span>
<span class="normal"><a href="#__codelineno-12-3"> 3</a></span>
<span class="normal"><a href="#__codelineno-12-4"> 4</a></span>
<span class="normal"><a href="#__codelineno-12-5"> 5</a></span>
<span class="normal"><a href="#__codelineno-12-6"> 6</a></span>
<span class="normal"><a href="#__codelineno-12-7"> 7</a></span>
<span class="normal"><a href="#__codelineno-12-8"> 8</a></span>
<span class="normal"><a href="#__codelineno-12-9"> 9</a></span>
<span class="normal"><a href="#__codelineno-12-10">10</a></span>
<span class="normal"><a href="#__codelineno-12-11">11</a></span>
<span class="normal"><a href="#__codelineno-12-12">12</a></span>
<span class="normal"><a href="#__codelineno-12-13">13</a></span>
<span class="normal"><a href="#__codelineno-12-14">14</a></span>
<span class="normal"><a href="#__codelineno-12-15">15</a></span>
<span class="normal"><a href="#__codelineno-12-16">16</a></span>
<span class="normal"><a href="#__codelineno-12-17">17</a></span>
<span class="normal"><a href="#__codelineno-12-18">18</a></span>
<span class="normal"><a href="#__codelineno-12-19">19</a></span>
<span class="normal"><a href="#__codelineno-12-20">20</a></span>
<span class="normal"><a href="#__codelineno-12-21">21</a></span>
<span class="normal"><a href="#__codelineno-12-22">22</a></span>
<span class="normal"><a href="#__codelineno-12-23">23</a></span>
<span class="normal"><a href="#__codelineno-12-24">24</a></span>
<span class="normal"><a href="#__codelineno-12-25">25</a></span>
<span class="normal"><a href="#__codelineno-12-26">26</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-12-1" name="__codelineno-12-1"></a>db.&lt;method&gt;<span class="o">()</span><span class="w"> </span><span class="c1"># database interaction</span>
<a id="__codelineno-12-2" name="__codelineno-12-2"></a>db.&lt;collection&gt;.&lt;method&gt;<span class="o">()</span><span class="w"> </span><span class="c1"># collection interaction</span>
<a id="__codelineno-12-3" name="__codelineno-12-3"></a>rs.&lt;method&gt;<span class="o">()</span><span class="p">;</span><span class="w"> </span><span class="c1"># replica set deployment and management</span>
<a id="__codelineno-12-4" name="__codelineno-12-4"></a>sh.&lt;method&gt;<span class="o">()</span><span class="p">;</span><span class="w"> </span><span class="c1"># sharded cluster deployment and management</span>
<a id="__codelineno-12-5" name="__codelineno-12-5"></a>
<a id="__codelineno-12-6" name="__codelineno-12-6"></a><span class="c1"># user management</span>
<a id="__codelineno-12-7" name="__codelineno-12-7"></a>db.createUser<span class="o">()</span>
<a id="__codelineno-12-8" name="__codelineno-12-8"></a>db.dropUser<span class="o">()</span>
<a id="__codelineno-12-9" name="__codelineno-12-9"></a>
<a id="__codelineno-12-10" name="__codelineno-12-10"></a><span class="c1"># collection management</span>
<a id="__codelineno-12-11" name="__codelineno-12-11"></a>db.renameCollection<span class="o">()</span>
<a id="__codelineno-12-12" name="__codelineno-12-12"></a>db.&lt;collection&gt;.createIndex<span class="o">()</span>
<a id="__codelineno-12-13" name="__codelineno-12-13"></a>db.&lt;collection&gt;.drop<span class="o">()</span>
<a id="__codelineno-12-14" name="__codelineno-12-14"></a>
<a id="__codelineno-12-15" name="__codelineno-12-15"></a><span class="c1"># database management</span>
<a id="__codelineno-12-16" name="__codelineno-12-16"></a>db.dropDatabase<span class="o">()</span>
<a id="__codelineno-12-17" name="__codelineno-12-17"></a>db.createCollection<span class="o">()</span>
<a id="__codelineno-12-18" name="__codelineno-12-18"></a>
<a id="__codelineno-12-19" name="__codelineno-12-19"></a><span class="c1"># database status</span>
<a id="__codelineno-12-20" name="__codelineno-12-20"></a>db.serverStatus<span class="o">()</span>
<a id="__codelineno-12-21" name="__codelineno-12-21"></a>
<a id="__codelineno-12-22" name="__codelineno-12-22"></a><span class="c1"># database command (underlying to shell helpers and drivers)</span>
<a id="__codelineno-12-23" name="__codelineno-12-23"></a>db.runCommand<span class="o">({</span><span class="w"> </span><span class="s2">&quot;&lt;COMMAND&gt;&quot;</span><span class="w"> </span><span class="o">})</span>
<a id="__codelineno-12-24" name="__codelineno-12-24"></a>
<a id="__codelineno-12-25" name="__codelineno-12-25"></a><span class="c1"># help</span>
<a id="__codelineno-12-26" name="__codelineno-12-26"></a>db.commandHelp<span class="o">(</span><span class="s2">&quot;&lt;command&gt;)</span>
</code></pre></div></td></tr></table></div>
<h3 id="logging">Logging</h3>
<p>The <strong>process log</strong> displays activity on the MongoDB instance and collects activities of various components:</p>
<p>Log Verbosity Level:</p>
<ul>
<li><code>-1</code>: Inherit from parent</li>
<li><code>0</code>: Default Verbosity (Information)</li>
<li><code>1 - 5</code>: Increases the verbosity up to Debug messages</li>
</ul>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-13-1">1</a></span>
<span class="normal"><a href="#__codelineno-13-2">2</a></span>
<span class="normal"><a href="#__codelineno-13-3">3</a></span>
<span class="normal"><a href="#__codelineno-13-4">4</a></span>
<span class="normal"><a href="#__codelineno-13-5">5</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-13-1" name="__codelineno-13-1"></a>db.getLogComponents<span class="o">()</span><span class="w"> </span><span class="c1"># get components and their verbosity</span>
<a id="__codelineno-13-2" name="__codelineno-13-2"></a>db.adminCommand<span class="o">({</span><span class="s2">&quot;getLog&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;scope&gt;&quot;</span><span class="o">})</span><span class="w"> </span><span class="c1"># retrieve logs (getLog must be run on admin db -&gt; adminCommand)</span>
<a id="__codelineno-13-3" name="__codelineno-13-3"></a>db.setLogLevel<span class="o">(</span>&lt;level&gt;,<span class="w"> </span><span class="s2">&quot;&lt;component&gt;&quot;</span><span class="o">)</span><span class="p">;</span><span class="w"> </span><span class="c1"># set log level (output is OLD verbosity levels)</span>
<a id="__codelineno-13-4" name="__codelineno-13-4"></a>
<a id="__codelineno-13-5" name="__codelineno-13-5"></a>tail<span class="w"> </span>-f<span class="w"> </span>/path/to/mongod.log<span class="w"> </span><span class="c1"># read end og log file</span>
</code></pre></div></td></tr></table></div>
<blockquote>
<p><strong>Note</strong>: Log Message Structure: <code>&lt;timestamp&gt; &lt;severity-level&gt; &lt;component&gt; &lt;connection&gt; &lt;event&gt; ...</code></p>
</blockquote>
<h3 id="database-profiling">Database Profiling</h3>
<p>Profiling Levels:</p>
<ul>
<li><code>0</code>: no profiling</li>
<li><code>1</code>: data on operations slower than <code>slowms</code> (default 100ms)</li>
<li><code>2</code>: data on all operations</li>
</ul>
<p>Events captured by the profiler:</p>
<ul>
<li>CRUD operations</li>
<li>Administrative operations</li>
<li>Configuration operations</li>
</ul>
<blockquote>
<p><strong>Note</strong>: Logs are saved in the <code>system.profile</code> <em>capped</em> collection.</p>
</blockquote>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-14-1">1</a></span>
<span class="normal"><a href="#__codelineno-14-2">2</a></span>
<span class="normal"><a href="#__codelineno-14-3">3</a></span>
<span class="normal"><a href="#__codelineno-14-4">4</a></span>
<span class="normal"><a href="#__codelineno-14-5">5</a></span>
<span class="normal"><a href="#__codelineno-14-6">6</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-14-1" name="__codelineno-14-1"></a>db.setProfilingLevel<span class="o">(</span>n<span class="o">)</span><span class="w"> </span><span class="c1"># set profiler level</span>
<a id="__codelineno-14-2" name="__codelineno-14-2"></a>db.setProfilingLevel<span class="o">(</span><span class="m">1</span>,<span class="w"> </span><span class="o">{</span><span class="w"> </span>slowms:<span class="w"> </span>&lt;ms&gt;<span class="w"> </span><span class="o">})</span>
<a id="__codelineno-14-3" name="__codelineno-14-3"></a>db.getProfilingStatus<span class="o">()</span><span class="w"> </span><span class="c1"># check profiler status</span>
<a id="__codelineno-14-4" name="__codelineno-14-4"></a>
<a id="__codelineno-14-5" name="__codelineno-14-5"></a>db.system.profile.find<span class="o">()</span>.limit<span class="o">(</span>n<span class="o">)</span>.sort<span class="o">(</span><span class="w"> </span><span class="o">{}</span><span class="w"> </span><span class="o">)</span>.pretty<span class="o">()</span><span class="w"> </span><span class="c1"># see logs</span>
<a id="__codelineno-14-6" name="__codelineno-14-6"></a>db.system.profile.find<span class="o">()</span>.limit<span class="o">(</span>n<span class="o">)</span>.sort<span class="o">(</span><span class="w"> </span><span class="o">{</span><span class="w"> </span>ts<span class="w"> </span>:<span class="w"> </span>-1<span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">)</span>.pretty<span class="o">()</span><span class="w"> </span><span class="c1"># sort by decreasing timestamp</span>
</code></pre></div></td></tr></table></div>
<h3 id="authentication">Authentication</h3>
<p>Client Authentication Mechanisms:</p>
<ul>
<li><strong>SCRAM</strong> (Default): Salted Challenge Response Authentication Mechanism</li>
<li><strong>X.509</strong>: <code>X.509</code> Certificate</li>
<li><strong>LADP</strong>: Lightweight Directory Access Protocol (Enterprise Only)</li>
<li><strong>KERBEROS</strong> (Enterprise Only)</li>
</ul>
<h3 id="authorization-role-based-access-control-rbac">Authorization: Role Based Access Control (RBAC)</h3>
<p>Each user has one or more <strong>Roles</strong>. Each role has one or more <strong>Privileges</strong>.<br />
A privilege represents a group of <em>actions</em> and the <em>resources</em> those actions apply to.</p>
<p>By default no user exists so the ONLY way to act is to connect locally to the server.<br />
This is the "localhost exception" and it closes after the <em>first</em> user is created.</p>
<blockquote>
<p><strong>Warn</strong>: Always create an admin user first (ideally with the <code>userAdmin</code> role)</p>
</blockquote>
<p>Role's <strong>Resources</strong>:</p>
<ul>
<li>specific database and collection: <code>{ "db": "&lt;database&gt;", "collection": "&lt;collection&gt;" }</code></li>
<li>all databases and collections: <code>{ "db": "", "collection": "" }</code></li>
<li>any databases and specific collection: <code>{ "db": "", "collection": "&lt;collections&gt;" }</code></li>
<li>specific database and any collection: <code>{ "db": "&lt;database&gt;", "collection": "" }</code></li>
<li>cluster resource: <code>{ "cluster": true }</code></li>
</ul>
<p>Role's <strong>Privileges</strong>: <code>{ resource: { &lt;resource&gt; }, actions: [ "&lt;action&gt;" ] }</code></p>
<p>A role can <em>inherit</em> from multiple others and can define <strong>network restrictions</strong> such as <em>Server Address</em> and <em>Client Source</em>.</p>
<p>Built-in Roles Groups and Names:</p>
<ul>
<li>Database User: <code>read</code>, <code>readWrite</code>, <code>readAnyDatabase</code>, <code>readWriteAnyDatabase</code></li>
<li>Database Administration: <code>dbAdmin</code>, <code>userAdmin</code>, <code>dbOwner</code>, <code>dbAdminAnyDatabase</code>, <code>userAdminAnyDatabase</code></li>
<li>Cluster Administration: <code>clusterAdmin</code>, <code>clusterManager</code>, <code>clusterMonitor</code>, <code>hostManager</code></li>
<li>Backup/Restore: <code>backup</code>, <code>restore</code></li>
<li>Super User: <code>root</code></li>
</ul>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-15-1"> 1</a></span>
<span class="normal"><a href="#__codelineno-15-2"> 2</a></span>
<span class="normal"><a href="#__codelineno-15-3"> 3</a></span>
<span class="normal"><a href="#__codelineno-15-4"> 4</a></span>
<span class="normal"><a href="#__codelineno-15-5"> 5</a></span>
<span class="normal"><a href="#__codelineno-15-6"> 6</a></span>
<span class="normal"><a href="#__codelineno-15-7"> 7</a></span>
<span class="normal"><a href="#__codelineno-15-8"> 8</a></span>
<span class="normal"><a href="#__codelineno-15-9"> 9</a></span>
<span class="normal"><a href="#__codelineno-15-10">10</a></span>
<span class="normal"><a href="#__codelineno-15-11">11</a></span>
<span class="normal"><a href="#__codelineno-15-12">12</a></span>
<span class="normal"><a href="#__codelineno-15-13">13</a></span>
<span class="normal"><a href="#__codelineno-15-14">14</a></span>
<span class="normal"><a href="#__codelineno-15-15">15</a></span>
<span class="normal"><a href="#__codelineno-15-16">16</a></span>
<span class="normal"><a href="#__codelineno-15-17">17</a></span>
<span class="normal"><a href="#__codelineno-15-18">18</a></span>
<span class="normal"><a href="#__codelineno-15-19">19</a></span>
<span class="normal"><a href="#__codelineno-15-20">20</a></span>
<span class="normal"><a href="#__codelineno-15-21">21</a></span>
<span class="normal"><a href="#__codelineno-15-22">22</a></span>
<span class="normal"><a href="#__codelineno-15-23">23</a></span>
<span class="normal"><a href="#__codelineno-15-24">24</a></span>
<span class="normal"><a href="#__codelineno-15-25">25</a></span>
<span class="normal"><a href="#__codelineno-15-26">26</a></span>
<span class="normal"><a href="#__codelineno-15-27">27</a></span>
<span class="normal"><a href="#__codelineno-15-28">28</a></span>
<span class="normal"><a href="#__codelineno-15-29">29</a></span>
<span class="normal"><a href="#__codelineno-15-30">30</a></span>
<span class="normal"><a href="#__codelineno-15-31">31</a></span>
<span class="normal"><a href="#__codelineno-15-32">32</a></span>
<span class="normal"><a href="#__codelineno-15-33">33</a></span>
<span class="normal"><a href="#__codelineno-15-34">34</a></span>
<span class="normal"><a href="#__codelineno-15-35">35</a></span>
<span class="normal"><a href="#__codelineno-15-36">36</a></span>
<span class="normal"><a href="#__codelineno-15-37">37</a></span>
<span class="normal"><a href="#__codelineno-15-38">38</a></span>
<span class="normal"><a href="#__codelineno-15-39">39</a></span>
<span class="normal"><a href="#__codelineno-15-40">40</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-15-1" name="__codelineno-15-1"></a>db.createUser<span class="o">({</span>
<a id="__codelineno-15-2" name="__codelineno-15-2"></a><span class="w"> </span>user:<span class="w"> </span><span class="s2">&quot;&lt;username&gt;&quot;</span>,
<a id="__codelineno-15-3" name="__codelineno-15-3"></a><span class="w"> </span>pwd:<span class="w"> </span><span class="s2">&quot;&lt;password&gt;&quot;</span>,
<a id="__codelineno-15-4" name="__codelineno-15-4"></a><span class="w"> </span>roles:<span class="w"> </span><span class="o">[</span><span class="w"> </span><span class="o">{</span><span class="w"> </span>role:<span class="w"> </span><span class="s2">&quot;&lt;role&gt;&quot;</span>,<span class="w"> </span>db:<span class="w"> </span><span class="s2">&quot;&lt;database&gt;&quot;</span><span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">]</span>
<a id="__codelineno-15-5" name="__codelineno-15-5"></a><span class="o">})</span>
<a id="__codelineno-15-6" name="__codelineno-15-6"></a>
<a id="__codelineno-15-7" name="__codelineno-15-7"></a>db.createRole<span class="o">({</span>
<a id="__codelineno-15-8" name="__codelineno-15-8"></a><span class="w"> </span>role:<span class="w"> </span><span class="s2">&quot;&lt;role&gt;&quot;</span>,
<a id="__codelineno-15-9" name="__codelineno-15-9"></a><span class="w"> </span>privileges:<span class="w"> </span><span class="o">[</span>
<a id="__codelineno-15-10" name="__codelineno-15-10"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span>resource:<span class="w"> </span><span class="o">{</span><span class="w"> </span>cluster:<span class="w"> </span><span class="nb">true</span><span class="w"> </span><span class="o">}</span>,<span class="w"> </span>actions:<span class="w"> </span><span class="o">[</span><span class="w"> </span><span class="s2">&quot;&lt;action&gt;&quot;</span>,<span class="w"> </span>...<span class="w"> </span><span class="o">]</span><span class="w"> </span><span class="o">}</span>,
<a id="__codelineno-15-11" name="__codelineno-15-11"></a><span class="w"> </span><span class="o">{</span>
<a id="__codelineno-15-12" name="__codelineno-15-12"></a><span class="w"> </span>resource:<span class="w"> </span><span class="o">{</span><span class="w"> </span>
<a id="__codelineno-15-13" name="__codelineno-15-13"></a><span class="w"> </span>db:<span class="w"> </span><span class="s2">&quot;&lt;database&gt;&quot;</span>,
<a id="__codelineno-15-14" name="__codelineno-15-14"></a><span class="w"> </span>collection:<span class="w"> </span><span class="s2">&quot;&lt;collection&gt;&quot;</span>
<a id="__codelineno-15-15" name="__codelineno-15-15"></a><span class="w"> </span><span class="o">}</span>,
<a id="__codelineno-15-16" name="__codelineno-15-16"></a><span class="w"> </span>actions:<span class="w"> </span><span class="o">[</span><span class="w"> </span><span class="s2">&quot;&lt;action&gt;&quot;</span>,<span class="w"> </span>...<span class="w"> </span><span class="o">]</span>
<a id="__codelineno-15-17" name="__codelineno-15-17"></a><span class="w"> </span><span class="o">}</span>,
<a id="__codelineno-15-18" name="__codelineno-15-18"></a><span class="w"> </span><span class="o">]</span>,
<a id="__codelineno-15-19" name="__codelineno-15-19"></a><span class="w"> </span>roles:<span class="w"> </span><span class="o">[</span>
<a id="__codelineno-15-20" name="__codelineno-15-20"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span>role:<span class="w"> </span><span class="s2">&quot;&lt;role&gt;&quot;</span>,<span class="w"> </span>db:<span class="w"> </span><span class="s2">&quot;&lt;database&gt;&quot;</span><span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="c1"># inherited permissions</span>
<a id="__codelineno-15-21" name="__codelineno-15-21"></a><span class="w"> </span><span class="o">]</span>
<a id="__codelineno-15-22" name="__codelineno-15-22"></a><span class="o">})</span>
<a id="__codelineno-15-23" name="__codelineno-15-23"></a>
<a id="__codelineno-15-24" name="__codelineno-15-24"></a><span class="c1"># add role to existing user</span>
<a id="__codelineno-15-25" name="__codelineno-15-25"></a>db.grantRolesToUser<span class="o">(</span>
<a id="__codelineno-15-26" name="__codelineno-15-26"></a><span class="w"> </span><span class="s2">&quot;&lt;user&gt;&quot;</span>,
<a id="__codelineno-15-27" name="__codelineno-15-27"></a><span class="w"> </span><span class="o">[</span>
<a id="__codelineno-15-28" name="__codelineno-15-28"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span>
<a id="__codelineno-15-29" name="__codelineno-15-29"></a><span class="w"> </span>role:<span class="w"> </span><span class="s2">&quot;&lt;role&gt;&quot;</span>
<a id="__codelineno-15-30" name="__codelineno-15-30"></a><span class="w"> </span>db:<span class="w"> </span><span class="s2">&quot;&lt;database&gt;&quot;</span>,
<a id="__codelineno-15-31" name="__codelineno-15-31"></a><span class="w"> </span>collection:<span class="w"> </span><span class="s2">&quot;&lt;collection&gt;&quot;</span>,
<a id="__codelineno-15-32" name="__codelineno-15-32"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-15-33" name="__codelineno-15-33"></a><span class="w"> </span><span class="o">]</span>
<a id="__codelineno-15-34" name="__codelineno-15-34"></a><span class="o">)</span>
<a id="__codelineno-15-35" name="__codelineno-15-35"></a>
<a id="__codelineno-15-36" name="__codelineno-15-36"></a><span class="c1"># show role privileges</span>
<a id="__codelineno-15-37" name="__codelineno-15-37"></a>db.runCommand<span class="o">({</span>
<a id="__codelineno-15-38" name="__codelineno-15-38"></a><span class="w"> </span>rolesInfo:<span class="w"> </span><span class="o">{</span><span class="w"> </span>db:<span class="w"> </span><span class="s2">&quot;&lt;database&gt;&quot;</span>,<span class="w"> </span>role:<span class="w"> </span><span class="s2">&quot;&lt;role&gt;&quot;</span><span class="w"> </span><span class="o">}</span>,
<a id="__codelineno-15-39" name="__codelineno-15-39"></a><span class="w"> </span>showPrivileges:<span class="w"> </span><span class="nb">true</span><span class="w"> </span>
<a id="__codelineno-15-40" name="__codelineno-15-40"></a><span class="o">})</span>
</code></pre></div></td></tr></table></div>
<h3 id="replica-set"><a href="https://docs.mongodb.com/manual/replication/">Replica set</a></h3>
<p>A <strong>replica set</strong> in MongoDB is a group of <code>mongod</code> processes that maintain the <code>same dataset</code>. Replica sets provide redundancy and high availability, and are the basis for all production deployments.</p>
<h3 id="sharding">Sharding</h3>
<p><strong>Sharding</strong> is a MongoDB concept through which big datasets are subdivided in smaller sets and distributed towards multiple instances of MongoDB.<br />
It's a technique used to improve the performances of large queries towards large quantities of data that require al lot of resources from the server.</p>
<p>A collection containing several documents is splitted in more smaller collections (<em>shards</em>)
Shards are implemented via cluster that are none other a group of MongoDB instances.</p>
<p>Shard components are:</p>
<ul>
<li>Shards (min 2), instances of MongoDB that contain a subset of the data</li>
<li>A config server, instance of MongoDB which contains metadata on the cluster, that is the set of instances that have the shard data.</li>
<li>A router (or <code>mongos</code>), instance of MongoDB used to redirect the user instructions from the client to the correct server.</li>
</ul>
<p><img alt="Shared Cluster" src="../img/mongodb_shared-cluster.avif" title="Components of a shared cluster" /></p>
<hr />
<h2 id="aggregation-framework"><a href="https://docs.mongodb.com/manual/reference/operator/aggregation-pipeline/">Aggregation Framework</a></h2>
<p>Sequence of operations applied to a collection as a <em>pipeline</em> to get a result: <code>db.collection.aggregate(pipeline, options)</code>.<br />
Each step of the pipeline acts on its inputs and not on the original data in the collection.</p>
<h3 id="variables">Variables</h3>
<p>Variable syntax in aggregations:</p>
<ul>
<li><code>$key</code>: field path</li>
<li><code>$$UPPERCASE</code>: system variable (e.g.: <code>$$CURRENT</code>)</li>
<li><code>$$foo</code>: user defined variable</li>
</ul>
<h3 id="match-aggregation-stage"><a href="https://www.mongodb.com/docs/manual/reference/operator/aggregation/match/" title="$match operator docs"><code>$match</code> Aggregation Stage</a></h3>
<p>Filters the documents to pass only the documents that match the specified condition(s) to the next pipeline stage.</p>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-16-1">1</a></span>
<span class="normal"><a href="#__codelineno-16-2">2</a></span>
<span class="normal"><a href="#__codelineno-16-3">3</a></span>
<span class="normal"><a href="#__codelineno-16-4">4</a></span>
<span class="normal"><a href="#__codelineno-16-5">5</a></span>
<span class="normal"><a href="#__codelineno-16-6">6</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-16-1" name="__codelineno-16-1"></a>db.&lt;collection&gt;.aggregate<span class="o">([</span><span class="w"> </span>
<a id="__codelineno-16-2" name="__codelineno-16-2"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$match</span><span class="s2">&quot;</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;&lt;query&gt;&quot;</span><span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">}</span>,
<a id="__codelineno-16-3" name="__codelineno-16-3"></a>
<a id="__codelineno-16-4" name="__codelineno-16-4"></a><span class="w"> </span><span class="c1"># key exists and is an array</span>
<a id="__codelineno-16-5" name="__codelineno-16-5"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="nv">$match</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;&lt;array-key&gt;&quot;</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="nv">$elemMatch</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="nv">$exists</span>:<span class="w"> </span><span class="nb">true</span><span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-16-6" name="__codelineno-16-6"></a><span class="o">})</span>
</code></pre></div></td></tr></table></div>
<blockquote>
<p><strong>Note</strong>: <code>$match</code> can contain the <code>$text</code> query operation but it <strong>must</strong> ber the <em>first</em> in a pipeline<br />
<strong>Note</strong>: <code>$match</code> cannot contain use <code>$where</code><br />
<strong>Note</strong>: <code>$match</code> uses the same syntax as <code>find()</code></p>
</blockquote>
<h3 id="project-aggregation-stage"><a href="https://www.mongodb.com/docs/manual/reference/operator/aggregation/project/" title="$project operator docs"><code>$project</code> Aggregation Stage</a></h3>
<p>Passes along the documents with the requested fields to the next stage in the pipeline. The specified fields can be existing fields from the input documents or newly computed fields.</p>
<p><code>$project</code> Array Expression Operators:</p>
<ul>
<li><a href="https://www.mongodb.com/docs/v4.4/reference/operator/aggregation/filter/" title="$filter operator docs"><code>$filter</code></a></li>
<li><a href="https://www.mongodb.com/docs/v4.4/reference/operator/aggregation/map/" title="$map operator docs"><code>$map</code></a></li>
<li><a href="https://www.mongodb.com/docs/v5.0/reference/operator/aggregation/reduce/" title="$reduce operator docs"><code>$reduce</code></a></li>
</ul>
<p><code>$project</code> Arithmetic Expression Operators:</p>
<ul>
<li><a href="https://www.mongodb.com/docs/v5.0/reference/operator/aggregation/max/" title="$max operator docs"><code>$max</code></a></li>
<li><a href="https://www.mongodb.com/docs/v5.0/reference/operator/aggregation/min/" title="$min operator docs"><code>$min</code></a></li>
<li><a href="https://www.mongodb.com/docs/v5.0/reference/operator/aggregation/sum/" title="$sum operator docs"><code>$sum</code></a></li>
<li><a href="https://www.mongodb.com/docs/v5.0/reference/operator/aggregation/avg/" title="$avg operator docs"><code>$avg</code></a></li>
</ul>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-17-1"> 1</a></span>
<span class="normal"><a href="#__codelineno-17-2"> 2</a></span>
<span class="normal"><a href="#__codelineno-17-3"> 3</a></span>
<span class="normal"><a href="#__codelineno-17-4"> 4</a></span>
<span class="normal"><a href="#__codelineno-17-5"> 5</a></span>
<span class="normal"><a href="#__codelineno-17-6"> 6</a></span>
<span class="normal"><a href="#__codelineno-17-7"> 7</a></span>
<span class="normal"><a href="#__codelineno-17-8"> 8</a></span>
<span class="normal"><a href="#__codelineno-17-9"> 9</a></span>
<span class="normal"><a href="#__codelineno-17-10">10</a></span>
<span class="normal"><a href="#__codelineno-17-11">11</a></span>
<span class="normal"><a href="#__codelineno-17-12">12</a></span>
<span class="normal"><a href="#__codelineno-17-13">13</a></span>
<span class="normal"><a href="#__codelineno-17-14">14</a></span>
<span class="normal"><a href="#__codelineno-17-15">15</a></span>
<span class="normal"><a href="#__codelineno-17-16">16</a></span>
<span class="normal"><a href="#__codelineno-17-17">17</a></span>
<span class="normal"><a href="#__codelineno-17-18">18</a></span>
<span class="normal"><a href="#__codelineno-17-19">19</a></span>
<span class="normal"><a href="#__codelineno-17-20">20</a></span>
<span class="normal"><a href="#__codelineno-17-21">21</a></span>
<span class="normal"><a href="#__codelineno-17-22">22</a></span>
<span class="normal"><a href="#__codelineno-17-23">23</a></span>
<span class="normal"><a href="#__codelineno-17-24">24</a></span>
<span class="normal"><a href="#__codelineno-17-25">25</a></span>
<span class="normal"><a href="#__codelineno-17-26">26</a></span>
<span class="normal"><a href="#__codelineno-17-27">27</a></span>
<span class="normal"><a href="#__codelineno-17-28">28</a></span>
<span class="normal"><a href="#__codelineno-17-29">29</a></span>
<span class="normal"><a href="#__codelineno-17-30">30</a></span>
<span class="normal"><a href="#__codelineno-17-31">31</a></span>
<span class="normal"><a href="#__codelineno-17-32">32</a></span>
<span class="normal"><a href="#__codelineno-17-33">33</a></span>
<span class="normal"><a href="#__codelineno-17-34">34</a></span>
<span class="normal"><a href="#__codelineno-17-35">35</a></span>
<span class="normal"><a href="#__codelineno-17-36">36</a></span>
<span class="normal"><a href="#__codelineno-17-37">37</a></span>
<span class="normal"><a href="#__codelineno-17-38">38</a></span>
<span class="normal"><a href="#__codelineno-17-39">39</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-17-1" name="__codelineno-17-1"></a>db.&lt;collection&gt;.aggregate<span class="o">([</span><span class="w"> </span>
<a id="__codelineno-17-2" name="__codelineno-17-2"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span>
<a id="__codelineno-17-3" name="__codelineno-17-3"></a><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$project</span><span class="s2">&quot;</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span>
<a id="__codelineno-17-4" name="__codelineno-17-4"></a><span class="w"> </span><span class="s2">&quot;_id&quot;</span>:<span class="w"> </span><span class="m">0</span>,<span class="w"> </span><span class="c1"># discard value</span>
<a id="__codelineno-17-5" name="__codelineno-17-5"></a><span class="w"> </span><span class="s2">&quot;&lt;key&gt;&quot;</span>:<span class="w"> </span><span class="m">1</span>,<span class="w"> </span><span class="c1"># keep value</span>
<a id="__codelineno-17-6" name="__codelineno-17-6"></a><span class="w"> </span><span class="s2">&quot;&lt;key&gt;&quot;</span>:<span class="w"> </span><span class="s2">&quot;</span>$<span class="s2">&lt;other-key&gt;&quot;</span><span class="w"> </span><span class="c1"># reassign or create field,</span>
<a id="__codelineno-17-7" name="__codelineno-17-7"></a><span class="w"> </span><span class="s2">&quot;&lt;key&gt;&quot;</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;&lt;expression&gt;&quot;</span><span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="c1"># calculate field value.</span>
<a id="__codelineno-17-8" name="__codelineno-17-8"></a>
<a id="__codelineno-17-9" name="__codelineno-17-9"></a><span class="w"> </span><span class="c1"># filter elements in an array</span>
<a id="__codelineno-17-10" name="__codelineno-17-10"></a><span class="w"> </span><span class="s2">&quot;&lt;key&gt;&quot;</span>:<span class="w"> </span><span class="o">{</span>
<a id="__codelineno-17-11" name="__codelineno-17-11"></a><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$filter</span><span class="s2">&quot;</span>:<span class="w"> </span><span class="o">{</span>
<a id="__codelineno-17-12" name="__codelineno-17-12"></a><span class="w"> </span><span class="s2">&quot;input&quot;</span>:<span class="w"> </span><span class="s2">&quot;</span>$<span class="s2">&lt;array-key&gt;&quot;</span>,
<a id="__codelineno-17-13" name="__codelineno-17-13"></a><span class="w"> </span><span class="s2">&quot;as&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;name-of-item&gt;&quot;</span>,
<a id="__codelineno-17-14" name="__codelineno-17-14"></a><span class="w"> </span><span class="s2">&quot;cond&quot;</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;&lt;bool-expression&gt;&quot;</span><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-17-15" name="__codelineno-17-15"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-17-16" name="__codelineno-17-16"></a><span class="w"> </span><span class="o">}</span>,
<a id="__codelineno-17-17" name="__codelineno-17-17"></a>
<a id="__codelineno-17-18" name="__codelineno-17-18"></a><span class="w"> </span><span class="c1"># transform array items</span>
<a id="__codelineno-17-19" name="__codelineno-17-19"></a><span class="w"> </span><span class="s2">&quot;&lt;key&gt;&quot;</span>:<span class="w"> </span><span class="o">{</span>
<a id="__codelineno-17-20" name="__codelineno-17-20"></a><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$map</span><span class="s2">&quot;</span>:<span class="w"> </span><span class="o">{</span>
<a id="__codelineno-17-21" name="__codelineno-17-21"></a><span class="w"> </span><span class="s2">&quot;input&quot;</span>:<span class="w"> </span><span class="s2">&quot;</span>$<span class="s2">&lt;array-key&gt;&quot;</span>,
<a id="__codelineno-17-22" name="__codelineno-17-22"></a><span class="w"> </span><span class="s2">&quot;as&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;item&gt;&quot;</span>,
<a id="__codelineno-17-23" name="__codelineno-17-23"></a><span class="w"> </span><span class="s2">&quot;in&quot;</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;&lt;expression&gt;&quot;</span><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-17-24" name="__codelineno-17-24"></a><span class="w"> </span><span class="c1"># $$&lt;item&gt; is the current item&#39;s value</span>
<a id="__codelineno-17-25" name="__codelineno-17-25"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-17-26" name="__codelineno-17-26"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-17-27" name="__codelineno-17-27"></a>
<a id="__codelineno-17-28" name="__codelineno-17-28"></a><span class="w"> </span><span class="c1"># apply expression to each element in an array and combine them</span>
<a id="__codelineno-17-29" name="__codelineno-17-29"></a><span class="w"> </span><span class="s2">&quot;&lt;key&gt;&quot;</span>:<span class="w"> </span><span class="o">{</span>
<a id="__codelineno-17-30" name="__codelineno-17-30"></a><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$reduce</span><span class="s2">&quot;</span>:<span class="w"> </span><span class="o">{</span>
<a id="__codelineno-17-31" name="__codelineno-17-31"></a><span class="w"> </span><span class="s2">&quot;input&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;array-key&gt;&quot;</span>,
<a id="__codelineno-17-32" name="__codelineno-17-32"></a><span class="w"> </span><span class="s2">&quot;initialValue&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;value&gt;&quot;</span>,
<a id="__codelineno-17-33" name="__codelineno-17-33"></a><span class="w"> </span><span class="s2">&quot;in&quot;</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;&lt;expression&gt;&quot;</span><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-17-34" name="__codelineno-17-34"></a><span class="w"> </span><span class="c1"># $$this is current document, $$value is current accumulated value</span>
<a id="__codelineno-17-35" name="__codelineno-17-35"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-17-36" name="__codelineno-17-36"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-17-37" name="__codelineno-17-37"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-17-38" name="__codelineno-17-38"></a><span class="w"> </span><span class="o">}</span><span class="w"> </span>
<a id="__codelineno-17-39" name="__codelineno-17-39"></a><span class="o">])</span>
</code></pre></div></td></tr></table></div>
<h3 id="addfields-aggregation-stage"><a href="https://www.mongodb.com/docs/manual/reference/operator/aggregation/addFields/" title="$addFields operator docs"><code>$addFields</code> Aggregation Stage</a></h3>
<p>Adds new fields to documents (can be result of computation).<br />
<code>$addFields</code> outputs documents that contain <em>all existing fields</em> from the input documents and newly added fields.</p>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-18-1">1</a></span>
<span class="normal"><a href="#__codelineno-18-2">2</a></span>
<span class="normal"><a href="#__codelineno-18-3">3</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-18-1" name="__codelineno-18-1"></a>db.&lt;collection&gt;.aggregate<span class="o">({</span>
<a id="__codelineno-18-2" name="__codelineno-18-2"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="nv">$addFields</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span>&lt;newField&gt;:<span class="w"> </span>&lt;expression&gt;,<span class="w"> </span>...<span class="w"> </span><span class="o">}</span><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-18-3" name="__codelineno-18-3"></a><span class="o">})</span>
</code></pre></div></td></tr></table></div>
<h3 id="group-aggregation-stage"><a href="https://www.mongodb.com/docs/manual/reference/operator/aggregation/group/" title="$group operator docs"><code>$group</code> Aggregation Stage</a></h3>
<p>The $<code>group</code> stage separates documents into groups according to a "group key". The output is one document for each unique group key.</p>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-19-1">1</a></span>
<span class="normal"><a href="#__codelineno-19-2">2</a></span>
<span class="normal"><a href="#__codelineno-19-3">3</a></span>
<span class="normal"><a href="#__codelineno-19-4">4</a></span>
<span class="normal"><a href="#__codelineno-19-5">5</a></span>
<span class="normal"><a href="#__codelineno-19-6">6</a></span>
<span class="normal"><a href="#__codelineno-19-7">7</a></span>
<span class="normal"><a href="#__codelineno-19-8">8</a></span>
<span class="normal"><a href="#__codelineno-19-9">9</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-19-1" name="__codelineno-19-1"></a>db.&lt;collection&gt;.aggregate<span class="o">([</span><span class="w"> </span>
<a id="__codelineno-19-2" name="__codelineno-19-2"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span>
<a id="__codelineno-19-3" name="__codelineno-19-3"></a><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$group</span><span class="s2">&quot;</span>:<span class="w"> </span><span class="o">{</span>
<a id="__codelineno-19-4" name="__codelineno-19-4"></a><span class="w"> </span><span class="s2">&quot;_id&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;expression&gt;&quot;</span>,<span class="w"> </span><span class="c1"># Group By Expression (Required)</span>
<a id="__codelineno-19-5" name="__codelineno-19-5"></a><span class="w"> </span><span class="s2">&quot;&lt;key-1&gt;&quot;</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;&lt;accumulator-1&gt;&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;expression-1&gt;&quot;</span><span class="w"> </span><span class="o">}</span>,
<a id="__codelineno-19-6" name="__codelineno-19-6"></a><span class="w"> </span>...
<a id="__codelineno-19-7" name="__codelineno-19-7"></a><span class="w"> </span><span class="o">}</span><span class="w"> </span>
<a id="__codelineno-19-8" name="__codelineno-19-8"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-19-9" name="__codelineno-19-9"></a><span class="o">])</span>
</code></pre></div></td></tr></table></div>
<h3 id="unwind-aggregation-stage"><a href="https://www.mongodb.com/docs/manual/reference/operator/aggregation/unwind/" title="$unwind operator docs"><code>$unwind</code> Aggregation Stage</a></h3>
<p>Deconstructs an array field from the input documents to output a document for each element.<br />
Each output document is the input document with the value of the array field replaced by the element</p>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-20-1"> 1</a></span>
<span class="normal"><a href="#__codelineno-20-2"> 2</a></span>
<span class="normal"><a href="#__codelineno-20-3"> 3</a></span>
<span class="normal"><a href="#__codelineno-20-4"> 4</a></span>
<span class="normal"><a href="#__codelineno-20-5"> 5</a></span>
<span class="normal"><a href="#__codelineno-20-6"> 6</a></span>
<span class="normal"><a href="#__codelineno-20-7"> 7</a></span>
<span class="normal"><a href="#__codelineno-20-8"> 8</a></span>
<span class="normal"><a href="#__codelineno-20-9"> 9</a></span>
<span class="normal"><a href="#__codelineno-20-10">10</a></span>
<span class="normal"><a href="#__codelineno-20-11">11</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-20-1" name="__codelineno-20-1"></a>db.&lt;collection&gt;.aggregate<span class="o">([</span><span class="w"> </span>
<a id="__codelineno-20-2" name="__codelineno-20-2"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$unwind</span><span class="s2">&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;array-key&gt;&quot;</span><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-20-3" name="__codelineno-20-3"></a>
<a id="__codelineno-20-4" name="__codelineno-20-4"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span>
<a id="__codelineno-20-5" name="__codelineno-20-5"></a><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$unwind</span><span class="s2">&quot;</span>:<span class="w"> </span><span class="o">{</span>
<a id="__codelineno-20-6" name="__codelineno-20-6"></a><span class="w"> </span><span class="s2">&quot;path&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;array-key&gt;&quot;</span>,<span class="w"> </span><span class="c1"># array to unwind</span>
<a id="__codelineno-20-7" name="__codelineno-20-7"></a><span class="w"> </span><span class="s2">&quot;includeArrayIndex&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;string&gt;&quot;</span>,<span class="w"> </span><span class="c1"># name of index field</span>
<a id="__codelineno-20-8" name="__codelineno-20-8"></a><span class="w"> </span><span class="s2">&quot;preserveNullAndEmptyArrays&quot;</span>:<span class="w"> </span>&lt;bool&gt;
<a id="__codelineno-20-9" name="__codelineno-20-9"></a><span class="w"> </span><span class="o">}</span><span class="w"> </span>
<a id="__codelineno-20-10" name="__codelineno-20-10"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-20-11" name="__codelineno-20-11"></a><span class="o">]</span>,<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;allowDiskUse&quot;</span>:<span class="w"> </span>&lt;bool&gt;<span class="w"> </span><span class="o">})</span>
</code></pre></div></td></tr></table></div>
<h3 id="count-aggregation-stage"><a href="https://www.mongodb.com/docs/manual/reference/operator/aggregation/count/" title="$count operator docs"><code>$count</code> Aggregation Stage</a></h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-21-1">1</a></span>
<span class="normal"><a href="#__codelineno-21-2">2</a></span>
<span class="normal"><a href="#__codelineno-21-3">3</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-21-1" name="__codelineno-21-1"></a>db.&lt;collection&gt;.aggregate<span class="o">([</span><span class="w"> </span>
<a id="__codelineno-21-2" name="__codelineno-21-2"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$count</span><span class="s2">&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;count-key&gt;&quot;</span><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-21-3" name="__codelineno-21-3"></a><span class="o">])</span>
</code></pre></div></td></tr></table></div>
<h3 id="sort-aggregation-stage"><a href="https://www.mongodb.com/docs/manual/reference/operator/aggregation/sort/" title="$sort operator docs"><code>$sort</code> Aggregation Stage</a></h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-22-1">1</a></span>
<span class="normal"><a href="#__codelineno-22-2">2</a></span>
<span class="normal"><a href="#__codelineno-22-3">3</a></span>
<span class="normal"><a href="#__codelineno-22-4">4</a></span>
<span class="normal"><a href="#__codelineno-22-5">5</a></span>
<span class="normal"><a href="#__codelineno-22-6">6</a></span>
<span class="normal"><a href="#__codelineno-22-7">7</a></span>
<span class="normal"><a href="#__codelineno-22-8">8</a></span>
<span class="normal"><a href="#__codelineno-22-9">9</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-22-1" name="__codelineno-22-1"></a>db.&lt;collection&gt;.aggregate<span class="o">([</span><span class="w"> </span>
<a id="__codelineno-22-2" name="__codelineno-22-2"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span>
<a id="__codelineno-22-3" name="__codelineno-22-3"></a><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$sort</span><span class="s2">&quot;</span>:<span class="w"> </span><span class="o">{</span><span class="w"> </span>
<a id="__codelineno-22-4" name="__codelineno-22-4"></a><span class="w"> </span><span class="s2">&quot;&lt;key-1&gt;&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;sort order&gt;&quot;</span>,<span class="w"> </span>
<a id="__codelineno-22-5" name="__codelineno-22-5"></a><span class="w"> </span><span class="s2">&quot;&lt;key-2&gt;&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;sort order&gt;&quot;</span>,<span class="w"> </span>
<a id="__codelineno-22-6" name="__codelineno-22-6"></a><span class="w"> </span>...<span class="w"> </span>
<a id="__codelineno-22-7" name="__codelineno-22-7"></a><span class="w"> </span><span class="o">}</span><span class="w"> </span>
<a id="__codelineno-22-8" name="__codelineno-22-8"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-22-9" name="__codelineno-22-9"></a><span class="o">]</span>,<span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;allowDiskUse&quot;</span>:<span class="w"> </span>&lt;bool&gt;<span class="w"> </span><span class="o">})</span>
</code></pre></div></td></tr></table></div>
<blockquote>
<p><strong>Note</strong>: can take advantage of indexes if early int the pipeline and before any <code>%project</code>, <code>$group</code> and <code>$unwind</code><br />
<strong>Note</strong>: By default <code>$sort</code> will use up to 10 MB of RAM. Setting <code>allowDiskUse: true</code> will allow for larger sorts</p>
</blockquote>
<h3 id="skip-aggregation-stage"><a href="https://www.mongodb.com/docs/manual/reference/operator/aggregation/skip/" title="$skip operator docs"><code>$skip</code> Aggregation Stage</a></h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-23-1">1</a></span>
<span class="normal"><a href="#__codelineno-23-2">2</a></span>
<span class="normal"><a href="#__codelineno-23-3">3</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-23-1" name="__codelineno-23-1"></a>db.&lt;collection&gt;.aggregate<span class="o">([</span><span class="w"> </span>
<a id="__codelineno-23-2" name="__codelineno-23-2"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$skip</span><span class="s2">&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;positive 64-bit integer&gt;&quot;</span><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-23-3" name="__codelineno-23-3"></a><span class="o">])</span>
</code></pre></div></td></tr></table></div>
<h3 id="limit-aggregation-stage"><a href="https://www.mongodb.com/docs/manual/reference/operator/aggregation/limit/" title="$limit operator docs"><code>$limit</code> Aggregation Stage</a></h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-24-1">1</a></span>
<span class="normal"><a href="#__codelineno-24-2">2</a></span>
<span class="normal"><a href="#__codelineno-24-3">3</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-24-1" name="__codelineno-24-1"></a>db.&lt;collection&gt;.aggregate<span class="o">([</span><span class="w"> </span>
<a id="__codelineno-24-2" name="__codelineno-24-2"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$limit</span><span class="s2">&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;positive 64-bit integer&gt;&quot;</span><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-24-3" name="__codelineno-24-3"></a><span class="o">])</span>
</code></pre></div></td></tr></table></div>
<h3 id="lookup-aggregation-stage"><a href="https://www.mongodb.com/docs/manual/reference/operator/aggregation/lookup/" title="$look operator docs"><code>$lookup</code> Aggregation Stage</a></h3>
<p>Performs a left outer join to a collection <em>in the same database</em> to filter in documents from the "joined" collection for processing.<br />
The <code>$lookup</code> stage adds a new array field to each input document. The new array field contains the matching documents from the "joined" collection.</p>
<blockquote>
<p><strong>Note</strong>: To combine elements from two different collections, use the <a href="https://www.mongodb.com/docs/manual/reference/operator/aggregation/unionWith/" title="$unionWith operator docs"><code>$unionWith</code></a> pipeline stage.</p>
</blockquote>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-25-1"> 1</a></span>
<span class="normal"><a href="#__codelineno-25-2"> 2</a></span>
<span class="normal"><a href="#__codelineno-25-3"> 3</a></span>
<span class="normal"><a href="#__codelineno-25-4"> 4</a></span>
<span class="normal"><a href="#__codelineno-25-5"> 5</a></span>
<span class="normal"><a href="#__codelineno-25-6"> 6</a></span>
<span class="normal"><a href="#__codelineno-25-7"> 7</a></span>
<span class="normal"><a href="#__codelineno-25-8"> 8</a></span>
<span class="normal"><a href="#__codelineno-25-9"> 9</a></span>
<span class="normal"><a href="#__codelineno-25-10">10</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-25-1" name="__codelineno-25-1"></a>db.&lt;collection&gt;.aggregate<span class="o">([</span><span class="w"> </span>
<a id="__codelineno-25-2" name="__codelineno-25-2"></a><span class="w"> </span><span class="o">{</span>
<a id="__codelineno-25-3" name="__codelineno-25-3"></a><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$lookup</span><span class="s2">&quot;</span>:<span class="w"> </span><span class="o">{</span>
<a id="__codelineno-25-4" name="__codelineno-25-4"></a><span class="w"> </span><span class="s2">&quot;from&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;foreign-collection&gt;&quot;</span>,
<a id="__codelineno-25-5" name="__codelineno-25-5"></a><span class="w"> </span><span class="s2">&quot;localField&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;key&gt;&quot;</span>,
<a id="__codelineno-25-6" name="__codelineno-25-6"></a><span class="w"> </span><span class="s2">&quot;foreignField&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;foreign-collection&gt;.&lt;key&gt;&quot;</span>,
<a id="__codelineno-25-7" name="__codelineno-25-7"></a><span class="w"> </span><span class="s2">&quot;as&quot;</span>:<span class="w"> </span><span class="s2">&quot;&lt;output array field&gt;&quot;</span>
<a id="__codelineno-25-8" name="__codelineno-25-8"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-25-9" name="__codelineno-25-9"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-25-10" name="__codelineno-25-10"></a><span class="o">])</span>
</code></pre></div></td></tr></table></div>
<h3 id="graphlookup-aggregation-stage"><a href="https://www.mongodb.com/docs/upcoming/reference/operator/aggregation/graphLookup/" title="$graphLookup operator docs"><code>$graphLookup</code> Aggregation Stage</a></h3>
<p>Performs a recursive search on a collection, with options for restricting the search by recursion depth and query filter.</p>
<p>The collection on which the aggregation is performed and the <code>from</code> collection can be the same (in-collection search) or different (cross-collection search)</p>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-26-1"> 1</a></span>
<span class="normal"><a href="#__codelineno-26-2"> 2</a></span>
<span class="normal"><a href="#__codelineno-26-3"> 3</a></span>
<span class="normal"><a href="#__codelineno-26-4"> 4</a></span>
<span class="normal"><a href="#__codelineno-26-5"> 5</a></span>
<span class="normal"><a href="#__codelineno-26-6"> 6</a></span>
<span class="normal"><a href="#__codelineno-26-7"> 7</a></span>
<span class="normal"><a href="#__codelineno-26-8"> 8</a></span>
<span class="normal"><a href="#__codelineno-26-9"> 9</a></span>
<span class="normal"><a href="#__codelineno-26-10">10</a></span>
<span class="normal"><a href="#__codelineno-26-11">11</a></span>
<span class="normal"><a href="#__codelineno-26-12">12</a></span>
<span class="normal"><a href="#__codelineno-26-13">13</a></span>
<span class="normal"><a href="#__codelineno-26-14">14</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-26-1" name="__codelineno-26-1"></a>db.&lt;collection&gt;.aggregate<span class="o">([</span>
<a id="__codelineno-26-2" name="__codelineno-26-2"></a><span class="w"> </span><span class="o">{</span>
<a id="__codelineno-26-3" name="__codelineno-26-3"></a><span class="w"> </span><span class="nv">$graphLookup</span>:<span class="w"> </span><span class="o">{</span>
<a id="__codelineno-26-4" name="__codelineno-26-4"></a><span class="w"> </span>from:<span class="w"> </span>&lt;collection&gt;,<span class="w"> </span><span class="c1"># starting collection of the search</span>
<a id="__codelineno-26-5" name="__codelineno-26-5"></a><span class="w"> </span>startWith:<span class="w"> </span>&lt;expression&gt;,<span class="w"> </span><span class="c1"># initial value(s) of search</span>
<a id="__codelineno-26-6" name="__codelineno-26-6"></a><span class="w"> </span>connectFromField:<span class="w"> </span>&lt;string&gt;,<span class="w"> </span><span class="c1"># source of the connection</span>
<a id="__codelineno-26-7" name="__codelineno-26-7"></a><span class="w"> </span>connectToField:<span class="w"> </span>&lt;string&gt;,<span class="w"> </span><span class="c1"># destination of the connection</span>
<a id="__codelineno-26-8" name="__codelineno-26-8"></a><span class="w"> </span>as:<span class="w"> </span>&lt;string&gt;,<span class="w"> </span><span class="c1"># array of found documents</span>
<a id="__codelineno-26-9" name="__codelineno-26-9"></a><span class="w"> </span>maxDepth:<span class="w"> </span>&lt;number&gt;,<span class="w"> </span><span class="c1"># recursive search depth limit (steps inside from collection)</span>
<a id="__codelineno-26-10" name="__codelineno-26-10"></a><span class="w"> </span>depthField:<span class="w"> </span>&lt;string&gt;,<span class="w"> </span><span class="c1"># field containing distance from start</span>
<a id="__codelineno-26-11" name="__codelineno-26-11"></a><span class="w"> </span>restrictSearchWithMatch:<span class="w"> </span>&lt;document&gt;<span class="w"> </span><span class="c1"># filter on found documents</span>
<a id="__codelineno-26-12" name="__codelineno-26-12"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-26-13" name="__codelineno-26-13"></a><span class="w"> </span><span class="o">}</span>
<a id="__codelineno-26-14" name="__codelineno-26-14"></a><span class="o">]</span>,<span class="w"> </span><span class="o">{</span><span class="w"> </span>allowDiskUse:<span class="w"> </span><span class="nb">true</span><span class="w"> </span><span class="o">})</span>
</code></pre></div></td></tr></table></div>
<blockquote>
<p><strong>Note</strong>: Having the <code>connectToField</code> indexed will improve search performance<br />
<strong>Warn</strong>: Can exceed the <code>100 Mb</code> memory limit even with <code>{ allowDiskUse: true }</code></p>
</blockquote>
<h3 id="sortbycount-aggregation-stage"><a href="https://www.mongodb.com/docs/upcoming/reference/operator/aggregation/sortByCount/" title="$sortByCount operator docs"><code>$sortByCount</code> Aggregation Stage</a></h3>
<p>Groups incoming documents based on the value of a specified expression, then computes the count of documents in each distinct group.</p>
<p>Each output document contains two fields: an <code>_id</code> field containing the distinct grouping value, and a <code>count</code> field containing the number of documents belonging to that grouping or category.</p>
<p>The documents are sorted by count in descending order.</p>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-27-1">1</a></span>
<span class="normal"><a href="#__codelineno-27-2">2</a></span>
<span class="normal"><a href="#__codelineno-27-3">3</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-27-1" name="__codelineno-27-1"></a>db.&lt;collection&gt;.aggregate<span class="o">([</span>
<a id="__codelineno-27-2" name="__codelineno-27-2"></a><span class="w"> </span><span class="o">{</span><span class="w"> </span><span class="nv">$sortByCount</span>:<span class="w"> </span>&lt;expression&gt;<span class="w"> </span><span class="o">}</span>
<a id="__codelineno-27-3" name="__codelineno-27-3"></a><span class="o">])</span>
</code></pre></div></td></tr></table></div>
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