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<div id="table-of-contents">
<ul class="nav nav-list side-nav well sidebar-nav-fixed">
<li class="main">
<a href="#serializer-relations">Serializer relations</a>
</li>
<li>
<a href="#inspecting-automatically-generated-relationships">Inspecting automatically generated relationships.</a>
</li>
<li class="main">
<a href="#api-reference">API Reference</a>
</li>
<li>
<a href="#stringrelatedfield">StringRelatedField</a>
</li>
<li>
<a href="#primarykeyrelatedfield">PrimaryKeyRelatedField</a>
</li>
<li>
<a href="#hyperlinkedrelatedfield">HyperlinkedRelatedField</a>
</li>
<li>
<a href="#slugrelatedfield">SlugRelatedField</a>
</li>
<li>
<a href="#hyperlinkedidentityfield">HyperlinkedIdentityField</a>
</li>
<li class="main">
<a href="#nested-relationships">Nested relationships</a>
</li>
<li>
<a href="#example">Example</a>
</li>
<li class="main">
<a href="#custom-relational-fields">Custom relational fields</a>
</li>
<li>
<a href="#example_1">Example</a>
</li>
<li class="main">
<a href="#further-notes">Further notes</a>
</li>
<li>
<a href="#the-queryset-argument">The queryset argument</a>
</li>
<li>
<a href="#reverse-relations">Reverse relations</a>
</li>
<li>
<a href="#generic-relationships">Generic relationships</a>
</li>
<li>
<a href="#manytomanyfields-with-a-through-model">ManyToManyFields with a Through Model</a>
</li>
<li>
<a href="#advanced-hyperlinked-fields">Advanced Hyperlinked fields</a>
</li>
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<a href="#third-party-packages">Third Party Packages</a>
</li>
<li>
<a href="#drf-nested-routers">DRF Nested Routers</a>
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</div>
<div id="main-content" class="span9">
<a class="github" href="https://github.com/tomchristie/django-rest-framework/tree/master/rest_framework/relations.py">
<span class="label label-info">relations.py</span>
</a>
<h1 id="serializer-relations">Serializer relations</h1>
<blockquote>
<p>Bad programmers worry about the code.
Good programmers worry about data structures and their relationships.</p>
<p>— <a href="http://lwn.net/Articles/193245/">Linus Torvalds</a></p>
</blockquote>
<p>Relational fields are used to represent model relationships. They can be applied to <code>ForeignKey</code>, <code>ManyToManyField</code> and <code>OneToOneField</code> relationships, as well as to reverse relationships, and custom relationships such as <code>GenericForeignKey</code>.</p>
<hr />
<p><strong>Note:</strong> The relational fields are declared in <code>relations.py</code>, but by convention you should import them from the <code>serializers</code> module, using <code>from rest_framework import serializers</code> and refer to fields as <code>serializers.<FieldName></code>.</p>
<hr />
<h4 id="inspecting-automatically-generated-relationships">Inspecting automatically generated relationships.</h4>
<p>When using the <code>ModelSerializer</code> class, serializer fields and relationships will be automatically generated for you. Inspecting these automatically generated fields can be a useful tool for determining how to customize the relationship style.</p>
<p>To do so, open the Django shell, using <code>python manage.py shell</code>, then import the serializer class, instantiate it, and print the object representation…</p>
<pre><code>>>> from myapp.serializers import AccountSerializer
>>> serializer = AccountSerializer()
>>> print repr(serializer) # Or `print(repr(serializer))` in Python 3.x.
AccountSerializer():
id = IntegerField(label='ID', read_only=True)
name = CharField(allow_blank=True, max_length=100, required=False)
owner = PrimaryKeyRelatedField(queryset=User.objects.all())
</code></pre>
<h1 id="api-reference">API Reference</h1>
<p>In order to explain the various types of relational fields, we'll use a couple of simple models for our examples. Our models will be for music albums, and the tracks listed on each album.</p>
<pre><code>class Album(models.Model):
album_name = models.CharField(max_length=100)
artist = models.CharField(max_length=100)
class Track(models.Model):
album = models.ForeignKey(Album, related_name='tracks')
order = models.IntegerField()
title = models.CharField(max_length=100)
duration = models.IntegerField()
class Meta:
unique_together = ('album', 'order')
ordering = ['order']
def __unicode__(self):
return '%d: %s' % (self.order, self.title)
</code></pre>
<h2 id="stringrelatedfield">StringRelatedField</h2>
<p><code>StringRelatedField</code> may be used to represent the target of the relationship using its <code>__unicode__</code> method.</p>
<p>For example, the following serializer.</p>
<pre><code>class AlbumSerializer(serializers.ModelSerializer):
tracks = serializers.StringRelatedField(many=True)
class Meta:
model = Album
fields = ('album_name', 'artist', 'tracks')
</code></pre>
<p>Would serialize to the following representation.</p>
<pre><code>{
'album_name': 'Things We Lost In The Fire',
'artist': 'Low',
'tracks': [
'1: Sunflower',
'2: Whitetail',
'3: Dinosaur Act',
...
]
}
</code></pre>
<p>This field is read only.</p>
<p><strong>Arguments</strong>:</p>
<ul>
<li><code>many</code> - If applied to a to-many relationship, you should set this argument to <code>True</code>.</li>
</ul>
<h2 id="primarykeyrelatedfield">PrimaryKeyRelatedField</h2>
<p><code>PrimaryKeyRelatedField</code> may be used to represent the target of the relationship using its primary key.</p>
<p>For example, the following serializer:</p>
<pre><code>class AlbumSerializer(serializers.ModelSerializer):
tracks = serializers.PrimaryKeyRelatedField(many=True, read_only=True)
class Meta:
model = Album
fields = ('album_name', 'artist', 'tracks')
</code></pre>
<p>Would serialize to a representation like this:</p>
<pre><code>{
'album_name': 'The Roots',
'artist': 'Undun',
'tracks': [
89,
90,
91,
...
]
}
</code></pre>
<p>By default this field is read-write, although you can change this behavior using the <code>read_only</code> flag.</p>
<p><strong>Arguments</strong>:</p>
<ul>
<li><code>queryset</code> - The queryset used for model instance lookups when validating the field input. Relationships must either set a queryset explicitly, or set <code>read_only=True</code>.</li>
<li><code>many</code> - If applied to a to-many relationship, you should set this argument to <code>True</code>.</li>
<li><code>allow_null</code> - If set to <code>True</code>, the field will accept values of <code>None</code> or the empty string for nullable relationships. Defaults to <code>False</code>.</li>
</ul>
<h2 id="hyperlinkedrelatedfield">HyperlinkedRelatedField</h2>
<p><code>HyperlinkedRelatedField</code> may be used to represent the target of the relationship using a hyperlink.</p>
<p>For example, the following serializer:</p>
<pre><code>class AlbumSerializer(serializers.ModelSerializer):
tracks = serializers.HyperlinkedRelatedField(
many=True,
read_only=True,
view_name='track-detail'
)
class Meta:
model = Album
fields = ('album_name', 'artist', 'tracks')
</code></pre>
<p>Would serialize to a representation like this:</p>
<pre><code>{
'album_name': 'Graceland',
'artist': 'Paul Simon',
'tracks': [
'http://www.example.com/api/tracks/45/',
'http://www.example.com/api/tracks/46/',
'http://www.example.com/api/tracks/47/',
...
]
}
</code></pre>
<p>By default this field is read-write, although you can change this behavior using the <code>read_only</code> flag.</p>
<p><strong>Arguments</strong>:</p>
<ul>
<li><code>view_name</code> - The view name that should be used as the target of the relationship. If you're using <a href="http://www.django-rest-framework.org/api-guide/routers#defaultrouter">the standard router classes</a> this will be a string with the format <code><modelname>-detail</code>. <strong>required</strong>.</li>
<li><code>queryset</code> - The queryset used for model instance lookups when validating the field input. Relationships must either set a queryset explicitly, or set <code>read_only=True</code>.</li>
<li><code>many</code> - If applied to a to-many relationship, you should set this argument to <code>True</code>.</li>
<li><code>allow_null</code> - If set to <code>True</code>, the field will accept values of <code>None</code> or the empty string for nullable relationships. Defaults to <code>False</code>.</li>
<li><code>lookup_field</code> - The field on the target that should be used for the lookup. Should correspond to a URL keyword argument on the referenced view. Default is <code>'pk'</code>.</li>
<li><code>lookup_url_kwarg</code> - The name of the keyword argument defined in the URL conf that corresponds to the lookup field. Defaults to using the same value as <code>lookup_field</code>.</li>
<li><code>format</code> - If using format suffixes, hyperlinked fields will use the same format suffix for the target unless overridden by using the <code>format</code> argument.</li>
</ul>
<h2 id="slugrelatedfield">SlugRelatedField</h2>
<p><code>SlugRelatedField</code> may be used to represent the target of the relationship using a field on the target.</p>
<p>For example, the following serializer:</p>
<pre><code>class AlbumSerializer(serializers.ModelSerializer):
tracks = serializers.SlugRelatedField(
many=True,
read_only=True,
slug_field='title'
)
class Meta:
model = Album
fields = ('album_name', 'artist', 'tracks')
</code></pre>
<p>Would serialize to a representation like this:</p>
<pre><code>{
'album_name': 'Dear John',
'artist': 'Loney Dear',
'tracks': [
'Airport Surroundings',
'Everything Turns to You',
'I Was Only Going Out',
...
]
}
</code></pre>
<p>By default this field is read-write, although you can change this behavior using the <code>read_only</code> flag.</p>
<p>When using <code>SlugRelatedField</code> as a read-write field, you will normally want to ensure that the slug field corresponds to a model field with <code>unique=True</code>.</p>
<p><strong>Arguments</strong>:</p>
<ul>
<li><code>slug_field</code> - The field on the target that should be used to represent it. This should be a field that uniquely identifies any given instance. For example, <code>username</code>. <strong>required</strong></li>
<li><code>queryset</code> - The queryset used for model instance lookups when validating the field input. Relationships must either set a queryset explicitly, or set <code>read_only=True</code>.</li>
<li><code>many</code> - If applied to a to-many relationship, you should set this argument to <code>True</code>.</li>
<li><code>allow_null</code> - If set to <code>True</code>, the field will accept values of <code>None</code> or the empty string for nullable relationships. Defaults to <code>False</code>.</li>
</ul>
<h2 id="hyperlinkedidentityfield">HyperlinkedIdentityField</h2>
<p>This field can be applied as an identity relationship, such as the <code>'url'</code> field on a HyperlinkedModelSerializer. It can also be used for an attribute on the object. For example, the following serializer:</p>
<pre><code>class AlbumSerializer(serializers.HyperlinkedModelSerializer):
track_listing = serializers.HyperlinkedIdentityField(view_name='track-list')
class Meta:
model = Album
fields = ('album_name', 'artist', 'track_listing')
</code></pre>
<p>Would serialize to a representation like this:</p>
<pre><code>{
'album_name': 'The Eraser',
'artist': 'Thom Yorke',
'track_listing': 'http://www.example.com/api/track_list/12/',
}
</code></pre>
<p>This field is always read-only.</p>
<p><strong>Arguments</strong>:</p>
<ul>
<li><code>view_name</code> - The view name that should be used as the target of the relationship. If you're using <a href="http://www.django-rest-framework.org/api-guide/routers#defaultrouter">the standard router classes</a> this will be a string with the format <code><model_name>-detail</code>. <strong>required</strong>.</li>
<li><code>lookup_field</code> - The field on the target that should be used for the lookup. Should correspond to a URL keyword argument on the referenced view. Default is <code>'pk'</code>.</li>
<li><code>lookup_url_kwarg</code> - The name of the keyword argument defined in the URL conf that corresponds to the lookup field. Defaults to using the same value as <code>lookup_field</code>.</li>
<li><code>format</code> - If using format suffixes, hyperlinked fields will use the same format suffix for the target unless overridden by using the <code>format</code> argument.</li>
</ul>
<hr />
<h1 id="nested-relationships">Nested relationships</h1>
<p>Nested relationships can be expressed by using serializers as fields.</p>
<p>If the field is used to represent a to-many relationship, you should add the <code>many=True</code> flag to the serializer field.</p>
<h2 id="example">Example</h2>
<p>For example, the following serializer:</p>
<pre><code>class TrackSerializer(serializers.ModelSerializer):
class Meta:
model = Track
fields = ('order', 'title')
class AlbumSerializer(serializers.ModelSerializer):
tracks = TrackSerializer(many=True, read_only=True)
class Meta:
model = Album
fields = ('album_name', 'artist', 'tracks')
</code></pre>
<p>Would serialize to a nested representation like this:</p>
<pre><code>{
'album_name': 'The Grey Album',
'artist': 'Danger Mouse',
'tracks': [
{'order': 1, 'title': 'Public Service Announcement'},
{'order': 2, 'title': 'What More Can I Say'},
{'order': 3, 'title': 'Encore'},
...
],
}
</code></pre>
<h1 id="custom-relational-fields">Custom relational fields</h1>
<p>To implement a custom relational field, you should override <code>RelatedField</code>, and implement the <code>.to_representation(self, value)</code> method. This method takes the target of the field as the <code>value</code> argument, and should return the representation that should be used to serialize the target. The <code>value</code> argument will typically be a model instance.</p>
<p>If you want to implement a read-write relational field, you must also implement the <code>.to_internal_value(self, data)</code> method.</p>
<h2 id="example_1">Example</h2>
<p>For example, we could define a relational field to serialize a track to a custom string representation, using its ordering, title, and duration.</p>
<pre><code>import time
class TrackListingField(serializers.RelatedField):
def to_representation(self, value):
duration = time.strftime('%M:%S', time.gmtime(value.duration))
return 'Track %d: %s (%s)' % (value.order, value.name, duration)
class AlbumSerializer(serializers.ModelSerializer):
tracks = TrackListingField(many=True)
class Meta:
model = Album
fields = ('album_name', 'artist', 'tracks')
</code></pre>
<p>This custom field would then serialize to the following representation.</p>
<pre><code>{
'album_name': 'Sometimes I Wish We Were an Eagle',
'artist': 'Bill Callahan',
'tracks': [
'Track 1: Jim Cain (04:39)',
'Track 2: Eid Ma Clack Shaw (04:19)',
'Track 3: The Wind and the Dove (04:34)',
...
]
}
</code></pre>
<hr />
<h1 id="further-notes">Further notes</h1>
<h2 id="the-queryset-argument">The <code>queryset</code> argument</h2>
<p>The <code>queryset</code> argument is only ever required for <em>writable</em> relationship field, in which case it is used for performing the model instance lookup, that maps from the primitive user input, into a model instance.</p>
<p>In version 2.x a serializer class could <em>sometimes</em> automatically determine the <code>queryset</code> argument <em>if</em> a <code>ModelSerializer</code> class was being used.</p>
<p>This behavior is now replaced with <em>always</em> using an explicit <code>queryset</code> argument for writable relational fields.</p>
<p>Doing so reduces the amount of hidden 'magic' that <code>ModelSerializer</code> provides, makes the behavior of the field more clear, and ensures that it is trivial to move between using the <code>ModelSerializer</code> shortcut, or using fully explicit <code>Serializer</code> classes.</p>
<h2 id="reverse-relations">Reverse relations</h2>
<p>Note that reverse relationships are not automatically included by the <code>ModelSerializer</code> and <code>HyperlinkedModelSerializer</code> classes. To include a reverse relationship, you must explicitly add it to the fields list. For example:</p>
<pre><code>class AlbumSerializer(serializers.ModelSerializer):
class Meta:
fields = ('tracks', ...)
</code></pre>
<p>You'll normally want to ensure that you've set an appropriate <code>related_name</code> argument on the relationship, that you can use as the field name. For example:</p>
<pre><code>class Track(models.Model):
album = models.ForeignKey(Album, related_name='tracks')
...
</code></pre>
<p>If you have not set a related name for the reverse relationship, you'll need to use the automatically generated related name in the <code>fields</code> argument. For example:</p>
<pre><code>class AlbumSerializer(serializers.ModelSerializer):
class Meta:
fields = ('track_set', ...)
</code></pre>
<p>See the Django documentation on <a href="https://docs.djangoproject.com/en/dev/topics/db/queries/#following-relationships-backward">reverse relationships</a> for more details.</p>
<h2 id="generic-relationships">Generic relationships</h2>
<p>If you want to serialize a generic foreign key, you need to define a custom field, to determine explicitly how you want serialize the targets of the relationship.</p>
<p>For example, given the following model for a tag, which has a generic relationship with other arbitrary models:</p>
<pre><code>class TaggedItem(models.Model):
"""
Tags arbitrary model instances using a generic relation.
See: https://docs.djangoproject.com/en/dev/ref/contrib/contenttypes/
"""
tag_name = models.SlugField()
content_type = models.ForeignKey(ContentType)
object_id = models.PositiveIntegerField()
tagged_object = GenericForeignKey('content_type', 'object_id')
def __unicode__(self):
return self.tag
</code></pre>
<p>And the following two models, which may be have associated tags:</p>
<pre><code>class Bookmark(models.Model):
"""
A bookmark consists of a URL, and 0 or more descriptive tags.
"""
url = models.URLField()
tags = GenericRelation(TaggedItem)
class Note(models.Model):
"""
A note consists of some text, and 0 or more descriptive tags.
"""
text = models.CharField(max_length=1000)
tags = GenericRelation(TaggedItem)
</code></pre>
<p>We could define a custom field that could be used to serialize tagged instances, using the type of each instance to determine how it should be serialized.</p>
<pre><code>class TaggedObjectRelatedField(serializers.RelatedField):
"""
A custom field to use for the `tagged_object` generic relationship.
"""
def to_representation(self, value):
"""
Serialize tagged objects to a simple textual representation.
"""
if isinstance(value, Bookmark):
return 'Bookmark: ' + value.url
elif isinstance(value, Note):
return 'Note: ' + value.text
raise Exception('Unexpected type of tagged object')
</code></pre>
<p>If you need the target of the relationship to have a nested representation, you can use the required serializers inside the <code>.to_representation()</code> method:</p>
<pre><code> def to_representation(self, value):
"""
Serialize bookmark instances using a bookmark serializer,
and note instances using a note serializer.
"""
if isinstance(value, Bookmark):
serializer = BookmarkSerializer(value)
elif isinstance(value, Note):
serializer = NoteSerializer(value)
else:
raise Exception('Unexpected type of tagged object')
return serializer.data
</code></pre>
<p>Note that reverse generic keys, expressed using the <code>GenericRelation</code> field, can be serialized using the regular relational field types, since the type of the target in the relationship is always known.</p>
<p>For more information see <a href="https://docs.djangoproject.com/en/dev/ref/contrib/contenttypes/#id1">the Django documentation on generic relations</a>.</p>
<h2 id="manytomanyfields-with-a-through-model">ManyToManyFields with a Through Model</h2>
<p>By default, relational fields that target a <code>ManyToManyField</code> with a
<code>through</code> model specified are set to read-only.</p>
<p>If you explicitly specify a relational field pointing to a
<code>ManyToManyField</code> with a through model, be sure to set <code>read_only</code>
to <code>True</code>.</p>
<h2 id="advanced-hyperlinked-fields">Advanced Hyperlinked fields</h2>
<p>If you have very specific requirements for the style of your hyperlinked relationships you can override <code>HyperlinkedRelatedField</code>.</p>
<p>There are two methods you'll need to override.</p>
<h4 id="get_urlself-obj-view_name-request-format">get_url(self, obj, view_name, request, format)</h4>
<p>This method should return the URL that corresponds to the given object.</p>
<p>May raise a <code>NoReverseMatch</code> if the <code>view_name</code> and <code>lookup_field</code>
attributes are not configured to correctly match the URL conf.</p>
<h4 id="get_objectself-queryset-view_name-view_args-view_kwargs">get_object(self, queryset, view_name, view_args, view_kwargs)</h4>
<p>This method should the object that corresponds to the matched URL conf arguments.</p>
<p>May raise an <code>ObjectDoesNotExist</code> exception.</p>
<h3 id="example_2">Example</h3>
<p>For example, if all your object URLs used both a account and a slug in the the URL to reference the object, you might create a custom field like this:</p>
<pre><code>class CustomHyperlinkedField(serializers.HyperlinkedRelatedField):
def get_url(self, obj, view_name, request, format):
kwargs = {'account': obj.account, 'slug': obj.slug}
return reverse(view_name, kwargs=kwargs, request=request, format=format)
def get_object(self, queryset, view_name, view_args, view_kwargs):
account = view_kwargs['account']
slug = view_kwargs['slug']
return queryset.get(account=account, slug=slug)
</code></pre>
<hr />
<h1 id="third-party-packages">Third Party Packages</h1>
<p>The following third party packages are also available.</p>
<h2 id="drf-nested-routers">DRF Nested Routers</h2>
<p>The <a href="https://github.com/alanjds/drf-nested-routers">drf-nested-routers package</a> provides routers and relationship fields for working with nested resources.</p>
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