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<!DOCTYPE html>
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<img src="../../images/droplet-white.svg" width="24" height="24">
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<span class="md-header-nav__parent">
Fluent
</span>
Relations
</span>
</div>
<div class="md-flex__cell md-flex__cell--shrink">
<label class="md-icon md-icon--search md-header-nav__button" for="search"></label>
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<div class="md-search__inner">
<form class="md-search__form" name="search">
<input type="text" class="md-search__input" name="query" placeholder="Search" accesskey="s" autocapitalize="off" autocorrect="off" autocomplete="off" spellcheck="false" data-md-component="query">
<label class="md-icon md-search__icon" for="search"></label>
</form>
<div class="md-search__output">
<div class="md-search__scrollwrap" data-md-scrollfix>
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<a href="https://github.com/vapor/documentation" title="Go to repository" class="md-source" data-md-source="github">
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<use xlink:href="#github" width="24" height="24"></use>
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</div>
<div class="md-source__repository">
GitHub
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</a>
</div>
</div>
</div>
</nav>
</header>
<div class="md-container">
<main class="md-main">
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<img src="../../images/droplet-white.svg">
</i>
Vapor Docs
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<div class="md-nav__source">
<a href="https://github.com/vapor/documentation" title="Go to repository" class="md-source" data-md-source="github">
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<svg viewBox="0 0 24 24" width="24" height="24">
<use xlink:href="#github" width="24" height="24"></use>
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</div>
<div class="md-source__repository">
GitHub
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</a>
</div>
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Overview
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Getting started
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<a href="../../getting-started/install-on-macos/" title="Install: macOS" class="md-nav__link">
Install: macOS
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<a href="../../getting-started/install-on-ubuntu/" title="Install: Ubuntu" class="md-nav__link">
Install: Ubuntu
</a>
</li>
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<a href="../../getting-started/toolbox/" title="Toolbox" class="md-nav__link">
Toolbox
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</li>
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<a href="../../getting-started/hello-world/" title="Hello, World" class="md-nav__link">
Hello, World
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</li>
<li class="md-nav__item">
<a href="../../getting-started/manual/" title="Manual" class="md-nav__link">
Manual
</a>
</li>
<li class="md-nav__item">
<a href="../../getting-started/xcode/" title="Xcode" class="md-nav__link">
Xcode
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</li>
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Vapor
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<nav class="md-nav" data-md-component="collapsible" data-md-level="1">
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Vapor
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<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../vapor/folder-structure/" title="Folder Structure" class="md-nav__link">
Folder Structure
</a>
</li>
<li class="md-nav__item">
<a href="../../vapor/droplet/" title="Droplet" class="md-nav__link">
Droplet
</a>
</li>
<li class="md-nav__item">
<a href="../../vapor/views/" title="Views" class="md-nav__link">
Views
</a>
</li>
<li class="md-nav__item">
<a href="../../vapor/controllers/" title="Controllers" class="md-nav__link">
Controllers
</a>
</li>
<li class="md-nav__item">
<a href="../../vapor/provider/" title="Provider" class="md-nav__link">
Provider
</a>
</li>
<li class="md-nav__item">
<a href="../../vapor/hash/" title="Hash" class="md-nav__link">
Hash
</a>
</li>
<li class="md-nav__item">
<a href="../../vapor/log/" title="Log" class="md-nav__link">
Log
</a>
</li>
<li class="md-nav__item">
<a href="../../vapor/commands/" title="Commands" class="md-nav__link">
Commands
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-toggle md-nav__toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4">
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Settings
</label>
<nav class="md-nav" data-md-component="collapsible" data-md-level="1">
<label class="md-nav__title" for="nav-4">
Settings
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../settings/config/" title="Config" class="md-nav__link">
Config
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-toggle md-nav__toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5">
<label class="md-nav__link" for="nav-5">
JSON
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<nav class="md-nav" data-md-component="collapsible" data-md-level="1">
<label class="md-nav__title" for="nav-5">
JSON
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../json/package/" title="Package" class="md-nav__link">
Package
</a>
</li>
<li class="md-nav__item">
<a href="../../json/overview/" title="Overview" class="md-nav__link">
Overview
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-toggle md-nav__toggle" data-md-toggle="nav-6" type="checkbox" id="nav-6">
<label class="md-nav__link" for="nav-6">
Routing
</label>
<nav class="md-nav" data-md-component="collapsible" data-md-level="1">
<label class="md-nav__title" for="nav-6">
Routing
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../routing/package/" title="Package" class="md-nav__link">
Package
</a>
</li>
<li class="md-nav__item">
<a href="../../routing/overview/" title="Overview" class="md-nav__link">
Overview
</a>
</li>
<li class="md-nav__item">
<a href="../../routing/parameters/" title="Parameters" class="md-nav__link">
Parameters
</a>
</li>
<li class="md-nav__item">
<a href="../../routing/group/" title="Group" class="md-nav__link">
Group
</a>
</li>
<li class="md-nav__item">
<a href="../../routing/collection/" title="Collection" class="md-nav__link">
Collection
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--active md-nav__item--nested">
<input class="md-toggle md-nav__toggle" data-md-toggle="nav-7" type="checkbox" id="nav-7" checked>
<label class="md-nav__link" for="nav-7">
Fluent
</label>
<nav class="md-nav" data-md-component="collapsible" data-md-level="1">
<label class="md-nav__title" for="nav-7">
Fluent
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../package/" title="Package" class="md-nav__link">
Package
</a>
</li>
<li class="md-nav__item">
<a href="../getting-started/" title="Getting Started" class="md-nav__link">
Getting Started
</a>
</li>
<li class="md-nav__item">
<a href="../model/" title="Model" class="md-nav__link">
Model
</a>
</li>
<li class="md-nav__item">
<a href="../database/" title="Database" class="md-nav__link">
Database
</a>
</li>
<li class="md-nav__item">
<a href="../query/" title="Query" class="md-nav__link">
Query
</a>
</li>
<li class="md-nav__item md-nav__item--active">
<input class="md-toggle md-nav__toggle" data-md-toggle="toc" type="checkbox" id="toc">
<label class="md-nav__link md-nav__link--active" for="toc">
Relations
</label>
<a href="./" title="Relations" class="md-nav__link md-nav__link--active">
Relations
</a>
<nav class="md-nav md-nav--secondary">
<label class="md-nav__title" for="toc">Table of contents</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="#one-to-many" title="One to Many" class="md-nav__link">
One to Many
</a>
<nav class="md-nav">
<ul class="md-nav__list">
<li class="md-nav__item">
<a href="#children" title="Children" class="md-nav__link">
Children
</a>
</li>
<li class="md-nav__item">
<a href="#parent" title="Parent" class="md-nav__link">
Parent
</a>
<nav class="md-nav">
<ul class="md-nav__list">
<li class="md-nav__item">
<a href="#migration" title="Migration" class="md-nav__link">
Migration
</a>
</li>
</ul>
</nav>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item">
<a href="#one-to-one" title="One to One" class="md-nav__link">
One to One
</a>
</li>
<li class="md-nav__item">
<a href="#many-to-many" title="Many to Many" class="md-nav__link">
Many to Many
</a>
<nav class="md-nav">
<ul class="md-nav__list">
<li class="md-nav__item">
<a href="#siblings" title="Siblings" class="md-nav__link">
Siblings
</a>
<nav class="md-nav">
<ul class="md-nav__list">
<li class="md-nav__item">
<a href="#migration_1" title="Migration" class="md-nav__link">
Migration
</a>
</li>
<li class="md-nav__item">
<a href="#add" title="Add" class="md-nav__link">
Add
</a>
</li>
<li class="md-nav__item">
<a href="#remove" title="Remove" class="md-nav__link">
Remove
</a>
</li>
<li class="md-nav__item">
<a href="#is-attached" title="Is Attached" class="md-nav__link">
Is Attached
</a>
</li>
<li class="md-nav__item">
<a href="#custom-through" title="Custom Through" class="md-nav__link">
Custom Through
</a>
</li>
</ul>
</nav>
</li>
</ul>
</nav>
</li>
</ul>
</nav>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-toggle md-nav__toggle" data-md-toggle="nav-8" type="checkbox" id="nav-8">
<label class="md-nav__link" for="nav-8">
Cache
</label>
<nav class="md-nav" data-md-component="collapsible" data-md-level="1">
<label class="md-nav__title" for="nav-8">
Cache
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../cache/package/" title="Package" class="md-nav__link">
Package
</a>
</li>
<li class="md-nav__item">
<a href="../../cache/overview/" title="Overview" class="md-nav__link">
Overview
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-toggle md-nav__toggle" data-md-toggle="nav-9" type="checkbox" id="nav-9">
<label class="md-nav__link" for="nav-9">
MySQL
</label>
<nav class="md-nav" data-md-component="collapsible" data-md-level="1">
<label class="md-nav__title" for="nav-9">
MySQL
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../mysql/package/" title="Package" class="md-nav__link">
Package
</a>
</li>
<li class="md-nav__item">
<a href="../../mysql/provider/" title="Provider" class="md-nav__link">
Provider
</a>
</li>
<li class="md-nav__item">
<a href="../../mysql/driver/" title="Driver" class="md-nav__link">
Driver
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-toggle md-nav__toggle" data-md-toggle="nav-10" type="checkbox" id="nav-10">
<label class="md-nav__link" for="nav-10">
Redis
</label>
<nav class="md-nav" data-md-component="collapsible" data-md-level="1">
<label class="md-nav__title" for="nav-10">
Redis
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../redis/package/" title="Package" class="md-nav__link">
Package
</a>
</li>
<li class="md-nav__item">
<a href="../../redis/provider/" title="Provider" class="md-nav__link">
Provider
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-toggle md-nav__toggle" data-md-toggle="nav-11" type="checkbox" id="nav-11">
<label class="md-nav__link" for="nav-11">
Auth
</label>
<nav class="md-nav" data-md-component="collapsible" data-md-level="1">
<label class="md-nav__title" for="nav-11">
Auth
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../auth/package/" title="Package" class="md-nav__link">
Package
</a>
</li>
<li class="md-nav__item">
<a href="../../auth/provider/" title="Provider" class="md-nav__link">
Provider
</a>
</li>
<li class="md-nav__item">
<a href="../../auth/getting-started/" title="Getting Started" class="md-nav__link">
Getting Started
</a>
</li>
<li class="md-nav__item">
<a href="../../auth/helper/" title="Helper" class="md-nav__link">
Helper
</a>
</li>
<li class="md-nav__item">
<a href="../../auth/password/" title="Password" class="md-nav__link">
Password
</a>
</li>
<li class="md-nav__item">
<a href="../../auth/persist/" title="Persist" class="md-nav__link">
Persist
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-toggle md-nav__toggle" data-md-toggle="nav-12" type="checkbox" id="nav-12">
<label class="md-nav__link" for="nav-12">
Sessions
</label>
<nav class="md-nav" data-md-component="collapsible" data-md-level="1">
<label class="md-nav__title" for="nav-12">
Sessions
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../sessions/sessions/" title="Sessions" class="md-nav__link">
Sessions
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-toggle md-nav__toggle" data-md-toggle="nav-13" type="checkbox" id="nav-13">
<label class="md-nav__link" for="nav-13">
HTTP
</label>
<nav class="md-nav" data-md-component="collapsible" data-md-level="1">
<label class="md-nav__title" for="nav-13">
HTTP
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../http/request/" title="Request" class="md-nav__link">
Request
</a>
</li>
<li class="md-nav__item">
<a href="../../http/response/" title="Response" class="md-nav__link">
Response
</a>
</li>
<li class="md-nav__item">
<a href="../../http/middleware/" title="Middleware" class="md-nav__link">
Middleware
</a>
</li>
<li class="md-nav__item">
<a href="../../http/body/" title="Body" class="md-nav__link">
Body
</a>
</li>
<li class="md-nav__item">
<a href="../../http/response-representable/" title="ResponseRepresentable" class="md-nav__link">
ResponseRepresentable
</a>
</li>
<li class="md-nav__item">
<a href="../../http/responder/" title="Responder" class="md-nav__link">
Responder
</a>
</li>
<li class="md-nav__item">
<a href="../../http/client/" title="Client" class="md-nav__link">
Client
</a>
</li>
<li class="md-nav__item">
<a href="../../http/server/" title="Server" class="md-nav__link">
Server
</a>
</li>
<li class="md-nav__item">
<a href="../../http/cors/" title="CORS" class="md-nav__link">
CORS
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-toggle md-nav__toggle" data-md-toggle="nav-14" type="checkbox" id="nav-14">
<label class="md-nav__link" for="nav-14">
Leaf
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<nav class="md-nav" data-md-component="collapsible" data-md-level="1">
<label class="md-nav__title" for="nav-14">
Leaf
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../leaf/package/" title="Package" class="md-nav__link">
Package
</a>
</li>
<li class="md-nav__item">
<a href="../../leaf/provider/" title="Provider" class="md-nav__link">
Provider
</a>
</li>
<li class="md-nav__item">
<a href="../../leaf/leaf/" title="Overview" class="md-nav__link">
Overview
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item">
<a href="../../validation/overview/" title="Validation" class="md-nav__link">
Validation
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<li class="md-nav__item md-nav__item--nested">
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<label class="md-nav__link" for="nav-16">
Node
</label>
<nav class="md-nav" data-md-component="collapsible" data-md-level="1">
<label class="md-nav__title" for="nav-16">
Node
</label>
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<li class="md-nav__item">
<a href="../../node/package/" title="Package" class="md-nav__link">
Package
</a>
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<a href="https://github.com/vapor/documentation/edit/master/2.0/docs/fluent/relations.md" title="Edit this page" class="md-icon md-content__edit">edit</a>
<h1 id="relations">Relations<a class="headerlink" href="#relations" title="Permanent link">&para;</a></h1>
<p>Fluent relations allow you to relate your models in three different ways:</p>
<table>
<thead>
<tr>
<th>Type</th>
<th>Relations</th>
</tr>
</thead>
<tbody>
<tr>
<td>One to One</td>
<td>Parent / Child</td>
</tr>
<tr>
<td>One to Many</td>
<td>Parent / Children</td>
</tr>
<tr>
<td>Many to Many</td>
<td>Siblings</td>
</tr>
</tbody>
</table>
<h2 id="one-to-many">One to Many<a class="headerlink" href="#one-to-many" title="Permanent link">&para;</a></h2>
<p>We'll start with one-to-many since it's the easiest type of relation to understand.</p>
<p>Take the following database schema:</p>
<p><code>users</code></p>
<table>
<thead>
<tr>
<th>id</th>
<th>name</th>
</tr>
</thead>
<tbody>
<tr>
<td>&lt;id type&gt;</td>
<td>string</td>
</tr>
</tbody>
</table>
<p><code>pets</code></p>
<table>
<thead>
<tr>
<th>id</th>
<th>name</th>
<th>user_id</th>
</tr>
</thead>
<tbody>
<tr>
<td>&lt;id type&gt;</td>
<td>string</td>
<td>&lt;id type&gt;</td>
</tr>
</tbody>
</table>
<div class="admonition seealso">
<p class="admonition-title">Seealso</p>
<p>Visit the <a href="../database/#preparations">database preparations</a> guide for more information
on how to create schema.</p>
</div>
<p>Here each pet has exactly one owner (a user) and each owner can have multiple pets.
This is a one-to-many relationship. One owner has many pets.</p>
<div class="admonition tip">
<p class="admonition-title">Tip</p>
<p>Use the <code>builder.foreignId()</code> to create foreign ids like <code>user_id</code>. This will automatically
create foreign key constraints and follow pre-set key naming conventions.</p>
</div>
<h3 id="children">Children<a class="headerlink" href="#children" title="Permanent link">&para;</a></h3>
<p>To access the user's pets, we will use the <code>Children</code> relation.</p>
<div class="codehilite"><pre><span></span><span class="kd">extension</span> <span class="nc">User</span> <span class="p">{</span>
<span class="kd">var</span> <span class="nv">pets</span><span class="p">:</span> <span class="n">Children</span><span class="p">&lt;</span><span class="n">User</span><span class="p">,</span> <span class="n">Pet</span><span class="p">&gt;</span> <span class="p">{</span>
<span class="k">return</span> <span class="n">children</span><span class="p">()</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>Imagine the children relation as <code>Children&lt;Parent, Child&gt;</code> or <code>Children&lt;From, To&gt;</code>.
Here we are relating <em>from</em> the user type <em>to</em> the pet type.</p>
<p>We can now use this relation to get all of the user's pets.</p>
<div class="codehilite"><pre><span></span><span class="kd">let</span> <span class="nv">pets</span> <span class="p">=</span> <span class="k">try</span> <span class="n">user</span><span class="p">.</span><span class="n">pets</span><span class="p">.</span><span class="n">all</span><span class="p">()</span> <span class="c1">// [Pet]</span>
</pre></div>
<p>This will create SQL similar to:</p>
<div class="codehilite"><pre><span></span><span class="k">SELECT</span> <span class="o">*</span> <span class="k">FROM</span> <span class="o">`</span><span class="n">pets</span><span class="o">`</span> <span class="k">WHERE</span> <span class="o">`</span><span class="n">user_id</span><span class="o">`</span> <span class="o">=</span> <span class="s1">&#39;...&#39;</span><span class="p">;</span>
</pre></div>
<p>Relations work similarly to <a href="../query/">queries</a>.</p>
<div class="codehilite"><pre><span></span><span class="kd">let</span> <span class="nv">pet</span> <span class="p">=</span> <span class="k">try</span> <span class="n">user</span><span class="p">.</span><span class="n">pets</span><span class="p">.</span><span class="bp">filter</span><span class="p">(</span><span class="s">&quot;name&quot;</span><span class="p">,</span> <span class="s">&quot;Spud&quot;</span><span class="p">).</span><span class="bp">first</span><span class="p">()</span>
</pre></div>
<h3 id="parent">Parent<a class="headerlink" href="#parent" title="Permanent link">&para;</a></h3>
<p>To access a pet's owner from the pet, we will use the <code>Parent</code> relation.</p>
<div class="codehilite"><pre><span></span><span class="kd">extension</span> <span class="nc">Pet</span> <span class="p">{</span>
<span class="kd">let</span> <span class="nv">userId</span><span class="p">:</span> <span class="n">Identifier</span>
<span class="p">...</span>
<span class="kd">var</span> <span class="nv">owner</span><span class="p">:</span> <span class="n">Parent</span><span class="p">&lt;</span><span class="n">Pet</span><span class="p">,</span> <span class="n">User</span><span class="p">&gt;</span> <span class="p">{</span>
<span class="k">return</span> <span class="n">parent</span><span class="p">(</span><span class="n">id</span><span class="p">:</span> <span class="n">userId</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>Imagine the parent relation as <code>Parent&lt;Child, Parent&gt;</code> or <code>Parent&lt;From, To&gt;</code>.
Here we are relating <em>from</em> the pet type <em>to</em> the parent type.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>Notice the <code>Parent</code> relation requires an identifier to be passed in.
Make sure to load this identifier in your model's <code>init(row:)</code> method.</p>
</div>
<p>We can now use this relation to get the pet's owner.</p>
<div class="codehilite"><pre><span></span><span class="kd">let</span> <span class="nv">owner</span> <span class="p">=</span> <span class="k">try</span> <span class="n">pet</span><span class="p">.</span><span class="n">owner</span><span class="p">.</span><span class="kr">get</span><span class="p">()</span> <span class="c1">// User?</span>
</pre></div>
<h4 id="migration">Migration<a class="headerlink" href="#migration" title="Permanent link">&para;</a></h4>
<p>Adding a parent identifier to the child table can be done using the <code>.parent()</code> method on
the schema builder.</p>
<div class="codehilite"><pre><span></span><span class="k">try</span> <span class="n">database</span><span class="p">.</span><span class="n">create</span><span class="p">(</span><span class="n">Pet</span><span class="p">.</span><span class="kc">self</span><span class="p">)</span> <span class="p">{</span> <span class="n">builder</span> <span class="k">in</span>
<span class="p">...</span>
<span class="n">builder</span><span class="p">.</span><span class="n">parent</span><span class="p">(</span><span class="n">User</span><span class="p">.</span><span class="kc">self</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
<h2 id="one-to-one">One to One<a class="headerlink" href="#one-to-one" title="Permanent link">&para;</a></h2>
<p>One-to-one relations work exactly the same as one-to-many relations. You can use the
code from the previous example and simply call <code>.first()</code> and all calls from the parent type.</p>
<p>However, you can add a convenience for doing this. Let's assume we wanted to change the previous
example from one-to-many to one-to-one.</p>
<div class="codehilite"><pre><span></span><span class="kd">extension</span> <span class="nc">User</span> <span class="p">{</span>
<span class="kd">func</span> <span class="nf">pet</span><span class="p">()</span> <span class="kr">throws</span> <span class="p">-&gt;</span> <span class="n">Pet</span><span class="p">?</span> <span class="p">{</span>
<span class="k">return</span> <span class="k">try</span> <span class="n">children</span><span class="p">().</span><span class="bp">first</span><span class="p">()</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<h2 id="many-to-many">Many to Many<a class="headerlink" href="#many-to-many" title="Permanent link">&para;</a></h2>
<p>Many to many relations require a table in between to store which model is related to which.
This table is called a pivot table.</p>
<p>You can use any entity you want as a pivot, but Fluent provides a default one called <code>Pivot</code>.</p>
<p>Take the following schema.</p>
<p><code>pets</code></p>
<table>
<thead>
<tr>
<th>id</th>
<th>name</th>
</tr>
</thead>
<tbody>
<tr>
<td>&lt;id type&gt;</td>
<td>string</td>
</tr>
</tbody>
</table>
<p><code>pet_toy</code></p>
<table>
<thead>
<tr>
<th>id</th>
<th>pet_id</th>
<th>toy_id</th>
</tr>
</thead>
<tbody>
<tr>
<td>&lt;id type&gt;</td>
<td>&lt;id type&gt;</td>
<td>&lt;id type&gt;</td>
</tr>
</tbody>
</table>
<p><code>toys</code></p>
<table>
<thead>
<tr>
<th>id</th>
<th>name</th>
</tr>
</thead>
<tbody>
<tr>
<td>&lt;id type&gt;</td>
<td>string</td>
</tr>
</tbody>
</table>
<p>Here each pet can own many toys and each toy can belong to many pets. This is a many-to-many relationship.</p>
<h3 id="siblings">Siblings<a class="headerlink" href="#siblings" title="Permanent link">&para;</a></h3>
<p>To represent this many-to-many relationship, we will use the <code>Siblings</code> relation.</p>
<div class="codehilite"><pre><span></span><span class="kd">extension</span> <span class="nc">Pet</span> <span class="p">{</span>
<span class="kd">var</span> <span class="nv">toys</span><span class="p">:</span> <span class="n">Siblings</span><span class="p">&lt;</span><span class="n">Pet</span><span class="p">,</span> <span class="n">Toy</span><span class="p">,</span> <span class="n">Pivot</span><span class="p">&lt;</span><span class="n">Pet</span><span class="p">,</span> <span class="n">Toy</span><span class="p">&gt;</span><span class="o">&gt;</span> <span class="p">{</span>
<span class="k">return</span> <span class="n">siblings</span><span class="p">()</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>Imagine the siblings relations as <code>Siblings&lt;From, To, Through&gt;</code>.
Here we are relating <em>from</em> the pet type <em>to</em> the toy type <em>through</em> the pet/toy pivot.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The generic syntax might look a little intimidating at first, but it allows for a very powerful API.</p>
</div>
<p>With this relation added on pets, we can fetch a pet's toys.</p>
<div class="codehilite"><pre><span></span><span class="kd">let</span> <span class="nv">toys</span> <span class="p">=</span> <span class="n">pet</span><span class="p">.</span><span class="n">toys</span><span class="p">.</span><span class="n">all</span><span class="p">()</span> <span class="c1">// [Toy]</span>
</pre></div>
<p>The siblings relation works similarly to <a href="../query/">queries</a> and parent/children relations. </p>
<h4 id="migration_1">Migration<a class="headerlink" href="#migration_1" title="Permanent link">&para;</a></h4>
<p>If you are using a <code>Pivot</code> type, you can simply add it to your Droplet's preparation array.</p>
<div class="codehilite"><pre><span></span><span class="n">drop</span><span class="p">.</span><span class="n">preparations</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="n">Pivot</span><span class="p">&lt;</span><span class="n">Pet</span><span class="p">,</span> <span class="n">Toy</span><span class="p">&gt;)</span>
</pre></div>
<p>If you are using a <code>Pivot</code> for your "through" model, it will also have methods for adding and removing models from the relation.</p>
<h4 id="add">Add<a class="headerlink" href="#add" title="Permanent link">&para;</a></h4>
<p>To add a new model to the relation, use the <code>.add()</code> method.</p>
<div class="codehilite"><pre><span></span><span class="k">try</span> <span class="n">pet</span><span class="p">.</span><span class="n">toys</span><span class="p">.</span><span class="n">add</span><span class="p">(</span><span class="n">toy</span><span class="p">)</span>
</pre></div>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The newly created pivot will be returned.</p>
</div>
<h4 id="remove">Remove<a class="headerlink" href="#remove" title="Permanent link">&para;</a></h4>
<p>To remove a model from being related, use the <code>.remove()</code> method.</p>
<div class="codehilite"><pre><span></span><span class="k">try</span> <span class="n">pet</span><span class="p">.</span><span class="n">toys</span><span class="p">.</span><span class="n">remove</span><span class="p">(</span><span class="n">toy</span><span class="p">)</span>
</pre></div>
<h4 id="is-attached">Is Attached<a class="headerlink" href="#is-attached" title="Permanent link">&para;</a></h4>
<p>To check if a model is related, use the <code>.isAttached()</code> method.</p>
<div class="codehilite"><pre><span></span><span class="k">if</span> <span class="k">try</span> <span class="n">pet</span><span class="p">.</span><span class="n">toys</span><span class="p">.</span><span class="n">isAttached</span><span class="p">(</span><span class="n">to</span><span class="p">:</span> <span class="n">toy</span><span class="p">)</span> <span class="p">{</span>
<span class="c1">// it is attached</span>
<span class="p">}</span>
</pre></div>
<h4 id="custom-through">Custom Through<a class="headerlink" href="#custom-through" title="Permanent link">&para;</a></h4>
<p>You can use any entity type as the "through" entity in your siblings relation. </p>
<div class="codehilite"><pre><span></span><span class="kd">extension</span> <span class="nc">User</span> <span class="p">{</span>
<span class="kd">var</span> <span class="nv">posts</span><span class="p">:</span> <span class="n">Siblings</span><span class="p">&lt;</span><span class="n">User</span><span class="p">,</span> <span class="n">Post</span><span class="p">,</span> <span class="n">Comment</span><span class="p">&gt;</span> <span class="p">{</span>
<span class="k">return</span> <span class="n">siblings</span><span class="p">()</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>In the above example we are pivoting on the comments entity to retreive all posts the user
has commented on.</p>
<p>As long as the "through" entity has a <code>user_id</code> and <code>post_id</code>, the siblings relation will work.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>If the <code>Comment</code>entity does not conform to <code>PivotProtocol</code>, the
<code>add</code>, <code>remove</code>, and <code>isAttached</code> methods will not be available.</p>
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