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<a href="https://github.com/vapor/documentation/blob/master/CONTRIBUTING.md" class="edit">✎ Edit on GitHub</a>
<h1 id="relation">Relation</h1>
<p>Relations allow foreign key based connections between database entities. This is common in SQL-based databases, but can also be used with NoSQL.</p>
<p>Fluent's relations are named as follows:</p>
<ul>
<li>Parent (BelongsTo)</li>
<li>Children (HasMany, HasOne)</li>
<li>Siblings (ManyToMany, BelongsToMany)</li>
</ul>
<h2 id="parent">Parent</h2>
<p>The parent relation should be called on an entity that has a foreign key to another entity. For example, assume the following schema:</p>
<pre><code>pets
- id
- owner_id
- name
- type
owner
- id
- name</code></pre>
<p>Here each pet can have one owner. To access that owner from the pet, call <code>.parent()</code>.</p>
<pre><code class="language-swift">let pet: Pet = ...
let owner = try pet.parent(pet.ownerId, Owner.self).get()</code></pre>
<p>The parent method requires the foreign key for the parent as well as the type.</p>
<h3 id="convenience">Convenience</h3>
<p>To make requesting a parent easier, a method can be added to the model.</p>
<pre><code class="language-swift">extension Pet {
func owner() throws -&gt; Parent&lt;Owner&gt; {
return try parent(ownerId)
}
}</code></pre>
<p>Since we are extending <code>Pet</code>, we no longer need to use <code>pet.</code> before the <code>ownerId</code>. Furthermore, because we are providing the type information about <code>Owner</code> in the return type of the method, we no longer need to pass that as an argument.</p>
<p>The <code>Parent&lt;T&gt;</code> type is a queryable object, meaning you could <code>delete()</code> the parent, <code>filter()</code>, etc.</p>
<pre><code class="language-swift">try pet.owner().delete()</code></pre>
<p>To fetch the parent, you must call <code>get()</code>.</p>
<pre><code class="language-swift">let owner = try pet.owner().get()</code></pre>
<h2 id="children">Children</h2>
<p><code>Children</code> are the opposite side of the <code>Parent</code> relationship. Assuming the schema from the previous example, the pets could be retrieved from an owner like so:</p>
<pre><code class="language-swift">let owner: Owner = ...
let pets = owner.children(Pet.self).all()</code></pre>
<p>Here only the type of child is required.</p>
<h3 id="convenience-1">Convenience</h3>
<p>Similarly to <code>Parent</code>, convenience methods can be added for children.</p>
<pre><code class="language-swift">extension Owner {
func pets() throws -&gt; Children&lt;Pet&gt; {
return try children()
}
}</code></pre>
<p>Since the type information is clear from the return value, <code>Pet.self</code> does not need to be passed to <code>children()</code>.</p>
<p>The <code>Children&lt;T&gt;</code> is also a queryable object like <code>Parent&lt;T&gt;</code>. You can call <code>first()</code>, <code>all()</code>, <code>filter()</code>, etc.</p>
<pre><code class="language-swift">let coolPets = try owner.pets().filter("type", .in, ["Dog", "Ferret"]).all()</code></pre>
<h2 id="siblings">Siblings</h2>
<p><code>Siblings</code> work differently from <code>Children</code> or <code>Parent</code> since they require a <code>Pivot</code>.</p>
<p>For an example, let's say we want to allow our pets to have multiple toys. But we also want the toys to be shared by multiple pets. We need a pivot entity for this.</p>
<pre><code>pets
- id
- type
- owner_id
toys
- id
- name
pets_toys
- id
- pet_id
- toy_id</code></pre>
<p>Here you can see the pivot entity, <code>pets_toys</code>, or <code>Pivot&lt;Pet, Toy&gt;</code>.</p>
<h3 id="convenience-2">Convenience</h3>
<p>Let's add the convenience methods to <code>Pet</code>.</p>
<pre><code class="language-swift">extension Pet {
func toys() throws -&gt; Siblings&lt;Toy&gt; {
return try siblings()
}
}</code></pre>
<p>And the opposite for <code>Toy</code>.</p>
<pre><code class="language-swift">extension Toy {
func pets() throws -&gt; Siblings&lt;Pet&gt; {
return try siblings()
}
}</code></pre>
<p>Now you are free to query pets and toys similarly to children.</p>
<pre><code class="language-swift">let pet: Pet = ...
let toys = pet.toys().all()</code></pre>
<p>To create a new many-to-many relationship you can do the following.</p>
<pre><code class="language-swift">
var toy: Toy = ... // Create a new toy
try toy.save() // Save the toy to the db
var pet: Pet = ... // Create a new pet
try pet.save() // Save the pet to the db
// Link them together in the db
var pivot = Pivot&lt;Toy, Pet&gt;(toy, pet) // Create the relationship
try pivot.save() // Save the relationship to the db</code></pre>
<h3 id="preparation">Preparation</h3>
<p>To prepare for a relationship with a <code>Pivot</code>, simply add the pivot to the <code>Droplet</code>'s preparations.</p>
<pre><code class="language-swift">let drop = Droplet()
drop.preparations += [
Toy.self,
Pet.self,
Pivot&lt;Toy, Pet&gt;.self
]</code></pre>
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