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479 lines
19 KiB
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479 lines
19 KiB
Markdown
source: relations.py
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---
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**Note**: This is the documentation for the **version 3.0** of REST framework. Documentation for [version 2.4](http://tomchristie.github.io/rest-framework-2-docs/) is also available.
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---
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# Serializer relations
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> Bad programmers worry about the code.
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> Good programmers worry about data structures and their relationships.
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>
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> — [Linus Torvalds][cite]
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Relational fields are used to represent model relationships. They can be applied to `ForeignKey`, `ManyToManyField` and `OneToOneField` relationships, as well as to reverse relationships, and custom relationships such as `GenericForeignKey`.
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---
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**Note:** The relational fields are declared in `relations.py`, but by convention you should import them from the `serializers` module, using `from rest_framework import serializers` and refer to fields as `serializers.<FieldName>`.
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---
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#### Inspecting automatically generated relationships.
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When using the `ModelSerializer` 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.
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To do so, open the Django shell, using `python manage.py shell`, then import the serializer class, instantiate it, and print the object representation…
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>>> from myapp.serializers import AccountSerializer
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>>> serializer = AccountSerializer()
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>>> print repr(serializer) # Or `print(repr(serializer))` in Python 3.x.
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AccountSerializer():
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id = IntegerField(label='ID', read_only=True)
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name = CharField(allow_blank=True, max_length=100, required=False)
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owner = PrimaryKeyRelatedField(queryset=User.objects.all())
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# API Reference
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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.
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class Album(models.Model):
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album_name = models.CharField(max_length=100)
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artist = models.CharField(max_length=100)
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class Track(models.Model):
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album = models.ForeignKey(Album, related_name='tracks')
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order = models.IntegerField()
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title = models.CharField(max_length=100)
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duration = models.IntegerField()
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class Meta:
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unique_together = ('album', 'order')
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order_by = 'order'
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def __unicode__(self):
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return '%d: %s' % (self.order, self.title)
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## StringRelatedField
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`StringRelatedField` may be used to represent the target of the relationship using its `__unicode__` method.
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For example, the following serializer.
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class AlbumSerializer(serializers.ModelSerializer):
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tracks = serializers.StringRelatedField(many=True)
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class Meta:
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model = Album
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fields = ('album_name', 'artist', 'tracks')
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Would serialize to the following representation.
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{
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'album_name': 'Things We Lost In The Fire',
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'artist': 'Low',
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'tracks': [
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'1: Sunflower',
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'2: Whitetail',
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'3: Dinosaur Act',
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...
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]
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}
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This field is read only.
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**Arguments**:
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* `many` - If applied to a to-many relationship, you should set this argument to `True`.
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## PrimaryKeyRelatedField
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`PrimaryKeyRelatedField` may be used to represent the target of the relationship using its primary key.
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For example, the following serializer:
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class AlbumSerializer(serializers.ModelSerializer):
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tracks = serializers.PrimaryKeyRelatedField(many=True, read_only=True)
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class Meta:
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model = Album
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fields = ('album_name', 'artist', 'tracks')
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Would serialize to a representation like this:
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{
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'album_name': 'The Roots',
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'artist': 'Undun',
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'tracks': [
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89,
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90,
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91,
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...
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]
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}
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By default this field is read-write, although you can change this behavior using the `read_only` flag.
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**Arguments**:
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* `queryset` - The queryset used for model instance lookups when validating the field input. Relationships must either set a queryset explicitly, or set `read_only=True`.
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* `many` - If applied to a to-many relationship, you should set this argument to `True`.
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* `allow_null` - If set to `True`, the field will accept values of `None` or the empty string for nullable relationships. Defaults to `False`.
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## HyperlinkedRelatedField
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`HyperlinkedRelatedField` may be used to represent the target of the relationship using a hyperlink.
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For example, the following serializer:
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class AlbumSerializer(serializers.ModelSerializer):
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tracks = serializers.HyperlinkedRelatedField(
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many=True,
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read_only=True,
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view_name='track-detail'
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)
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class Meta:
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model = Album
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fields = ('album_name', 'artist', 'tracks')
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Would serialize to a representation like this:
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{
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'album_name': 'Graceland',
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'artist': 'Paul Simon',
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'tracks': [
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'http://www.example.com/api/tracks/45/',
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'http://www.example.com/api/tracks/46/',
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'http://www.example.com/api/tracks/47/',
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...
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]
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}
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By default this field is read-write, although you can change this behavior using the `read_only` flag.
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**Arguments**:
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* `view_name` - The view name that should be used as the target of the relationship. If you're using [the standard router classes][routers] this will be a string with the format `<modelname>-detail`. **required**.
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* `queryset` - The queryset used for model instance lookups when validating the field input. Relationships must either set a queryset explicitly, or set `read_only=True`.
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* `many` - If applied to a to-many relationship, you should set this argument to `True`.
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* `allow_null` - If set to `True`, the field will accept values of `None` or the empty string for nullable relationships. Defaults to `False`.
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* `lookup_field` - 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 `'pk'`.
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* `lookup_url_kwarg` - The name of the keyword argument defined in the URL conf that corresponds to the lookup field. Defaults to using the same value as `lookup_field`.
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* `format` - If using format suffixes, hyperlinked fields will use the same format suffix for the target unless overridden by using the `format` argument.
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## SlugRelatedField
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`SlugRelatedField` may be used to represent the target of the relationship using a field on the target.
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For example, the following serializer:
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class AlbumSerializer(serializers.ModelSerializer):
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tracks = serializers.SlugRelatedField(
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many=True,
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read_only=True,
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slug_field='title'
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)
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class Meta:
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model = Album
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fields = ('album_name', 'artist', 'tracks')
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Would serialize to a representation like this:
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{
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'album_name': 'Dear John',
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'artist': 'Loney Dear',
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'tracks': [
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'Airport Surroundings',
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'Everything Turns to You',
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'I Was Only Going Out',
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...
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]
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}
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By default this field is read-write, although you can change this behavior using the `read_only` flag.
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When using `SlugRelatedField` as a read-write field, you will normally want to ensure that the slug field corresponds to a model field with `unique=True`.
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**Arguments**:
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* `slug_field` - 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, `username`. **required**
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* `queryset` - The queryset used for model instance lookups when validating the field input. Relationships must either set a queryset explicitly, or set `read_only=True`.
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* `many` - If applied to a to-many relationship, you should set this argument to `True`.
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* `allow_null` - If set to `True`, the field will accept values of `None` or the empty string for nullable relationships. Defaults to `False`.
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## HyperlinkedIdentityField
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This field can be applied as an identity relationship, such as the `'url'` field on a HyperlinkedModelSerializer. It can also be used for an attribute on the object. For example, the following serializer:
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class AlbumSerializer(serializers.HyperlinkedModelSerializer):
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track_listing = serializers.HyperlinkedIdentityField(view_name='track-list')
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class Meta:
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model = Album
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fields = ('album_name', 'artist', 'track_listing')
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Would serialize to a representation like this:
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{
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'album_name': 'The Eraser',
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'artist': 'Thom Yorke',
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'track_listing': 'http://www.example.com/api/track_list/12/',
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}
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This field is always read-only.
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**Arguments**:
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* `view_name` - The view name that should be used as the target of the relationship. If you're using [the standard router classes][routers] this will be a string with the format `<model_name>-detail`. **required**.
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* `lookup_field` - 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 `'pk'`.
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* `lookup_url_kwarg` - The name of the keyword argument defined in the URL conf that corresponds to the lookup field. Defaults to using the same value as `lookup_field`.
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* `format` - If using format suffixes, hyperlinked fields will use the same format suffix for the target unless overridden by using the `format` argument.
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---
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# Nested relationships
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Nested relationships can be expressed by using serializers as fields.
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If the field is used to represent a to-many relationship, you should add the `many=True` flag to the serializer field.
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## Example
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For example, the following serializer:
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class TrackSerializer(serializers.ModelSerializer):
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class Meta:
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model = Track
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fields = ('order', 'title')
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class AlbumSerializer(serializers.ModelSerializer):
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tracks = TrackSerializer(many=True, read_only=True)
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class Meta:
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model = Album
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fields = ('album_name', 'artist', 'tracks')
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Would serialize to a nested representation like this:
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{
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'album_name': 'The Grey Album',
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'artist': 'Danger Mouse',
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'tracks': [
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{'order': 1, 'title': 'Public Service Announcement'},
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{'order': 2, 'title': 'What More Can I Say'},
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{'order': 3, 'title': 'Encore'},
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...
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],
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}
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# Custom relational fields
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To implement a custom relational field, you should override `RelatedField`, and implement the `.to_representation(self, value)` method. This method takes the target of the field as the `value` argument, and should return the representation that should be used to serialize the target. The `value` argument will typically be a model instance.
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If you want to implement a read-write relational field, you must also implement the `.to_internal_value(self, data)` method.
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## Example
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For, example, we could define a relational field, to serialize a track to a custom string representation, using its ordering, title, and duration.
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import time
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class TrackListingField(serializers.RelatedField):
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def to_representation(self, value):
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duration = time.strftime('%M:%S', time.gmtime(value.duration))
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return 'Track %d: %s (%s)' % (value.order, value.name, duration)
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class AlbumSerializer(serializers.ModelSerializer):
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tracks = TrackListingField(many=True)
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class Meta:
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model = Album
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fields = ('album_name', 'artist', 'tracks')
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This custom field would then serialize to the following representation.
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{
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'album_name': 'Sometimes I Wish We Were an Eagle',
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'artist': 'Bill Callahan',
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'tracks': [
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'Track 1: Jim Cain (04:39)',
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'Track 2: Eid Ma Clack Shaw (04:19)',
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'Track 3: The Wind and the Dove (04:34)',
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...
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]
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}
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---
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# Further notes
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## The `queryset` argument
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The `queryset` argument is only ever required for *writable* 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.
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In version 2.x a serializer class could *sometimes* automatically determine the `queryset` argument *if* a `ModelSerializer` class was being used.
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This behavior is now replaced with *always* using an explicit `queryset` argument for writable relational fields.
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Doing so reduces the amount of hidden 'magic' that `ModelSerializer` provides, makes the behavior of the field more clear, and ensures that it is trivial to move between using the `ModelSerializer` shortcut, or using fully explicit `Serializer` classes.
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## Reverse relations
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Note that reverse relationships are not automatically included by the `ModelSerializer` and `HyperlinkedModelSerializer` classes. To include a reverse relationship, you must explicitly add it to the fields list. For example:
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class AlbumSerializer(serializers.ModelSerializer):
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class Meta:
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fields = ('tracks', ...)
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You'll normally want to ensure that you've set an appropriate `related_name` argument on the relationship, that you can use as the field name. For example:
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class Track(models.Model):
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album = models.ForeignKey(Album, related_name='tracks')
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...
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If you have not set a related name for the reverse relationship, you'll need to use the automatically generated related name in the `fields` argument. For example:
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class AlbumSerializer(serializers.ModelSerializer):
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class Meta:
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fields = ('track_set', ...)
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See the Django documentation on [reverse relationships][reverse-relationships] for more details.
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## Generic relationships
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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.
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For example, given the following model for a tag, which has a generic relationship with other arbitrary models:
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class TaggedItem(models.Model):
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"""
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Tags arbitrary model instances using a generic relation.
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See: https://docs.djangoproject.com/en/dev/ref/contrib/contenttypes/
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"""
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tag_name = models.SlugField()
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content_type = models.ForeignKey(ContentType)
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object_id = models.PositiveIntegerField()
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tagged_object = GenericForeignKey('content_type', 'object_id')
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def __unicode__(self):
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return self.tag
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And the following two models, which may be have associated tags:
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class Bookmark(models.Model):
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"""
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A bookmark consists of a URL, and 0 or more descriptive tags.
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"""
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url = models.URLField()
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tags = GenericRelation(TaggedItem)
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class Note(models.Model):
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"""
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A note consists of some text, and 0 or more descriptive tags.
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"""
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text = models.CharField(max_length=1000)
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tags = GenericRelation(TaggedItem)
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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.
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class TaggedObjectRelatedField(serializers.RelatedField):
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"""
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A custom field to use for the `tagged_object` generic relationship.
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"""
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def to_representation(self, value):
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"""
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Serialize tagged objects to a simple textual representation.
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"""
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if isinstance(value, Bookmark):
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return 'Bookmark: ' + value.url
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elif isinstance(value, Note):
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return 'Note: ' + value.text
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raise Exception('Unexpected type of tagged object')
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If you need the target of the relationship to have a nested representation, you can use the required serializers inside the `.to_representation()` method:
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def to_representation(self, value):
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"""
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Serialize bookmark instances using a bookmark serializer,
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and note instances using a note serializer.
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"""
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if isinstance(value, Bookmark):
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serializer = BookmarkSerializer(value)
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elif isinstance(value, Note):
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serializer = NoteSerializer(value)
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else:
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raise Exception('Unexpected type of tagged object')
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return serializer.data
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Note that reverse generic keys, expressed using the `GenericRelation` field, can be serialized using the regular relational field types, since the type of the target in the relationship is always known.
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For more information see [the Django documentation on generic relations][generic-relations].
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## ManyToManyFields with a Through Model
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By default, relational fields that target a ``ManyToManyField`` with a
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``through`` model specified are set to read-only.
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If you explicitly specify a relational field pointing to a
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``ManyToManyField`` with a through model, be sure to set ``read_only``
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to ``True``.
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## Advanced Hyperlinked fields
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If you have very specific requirements for the style of your hyperlinked relationships you can override `HyperlinkedRelatedField`.
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There are two methods you'll need to override.
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#### get_url(self, obj, view_name, request, format)
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This method should return the URL that corresponds to the given object.
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May raise a `NoReverseMatch` if the `view_name` and `lookup_field`
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attributes are not configured to correctly match the URL conf.
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#### get_object(self, queryset, view_name, view_args, view_kwargs)
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This method should the object that corresponds to the matched URL conf arguments.
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May raise an `ObjectDoesNotExist` exception.
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### Example
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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:
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class CustomHyperlinkedField(serializers.HyperlinkedRelatedField):
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def get_url(self, obj, view_name, request, format):
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kwargs = {'account': obj.account, 'slug': obj.slug}
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return reverse(view_name, kwargs=kwargs, request=request, format=format)
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def get_object(self, queryset, view_name, view_args, view_kwargs):
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account = view_kwargs['account']
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slug = view_kwargs['slug']
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return queryset.get(account=account, slug=slug)
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---
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# Third Party Packages
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The following third party packages are also available.
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## DRF Nested Routers
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The [drf-nested-routers package][drf-nested-routers] provides routers and relationship fields for working with nested resources.
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[cite]: http://lwn.net/Articles/193245/
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[reverse-relationships]: https://docs.djangoproject.com/en/dev/topics/db/queries/#following-relationships-backward
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[routers]: http://www.django-rest-framework.org/api-guide/routers#defaultrouter
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[generic-relations]: https://docs.djangoproject.com/en/dev/ref/contrib/contenttypes/#id1
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[2.2-announcement]: ../topics/2.2-announcement.md
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[drf-nested-routers]: https://github.com/alanjds/drf-nested-routers
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