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relations.py

Serializer relations

Bad programmers worry about the code. Good programmers worry about data structures and their relationships.

— Linus Torvalds

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.

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>.

API Reference

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.

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')
        order_by = 'order'

    def __unicode__(self):
        return '%d: %s' % (self.order, self.title)

RelatedField

RelatedField may be used to represent the target of the relationship using its __unicode__ method.

For example, the following serializer.

class AlbumSerializer(serializers.ModelSerializer):
    tracks = serializers.RelatedField(many=True)

    class Meta:
        model = Album
        fields = ('album_name', 'artist', 'tracks')

Would serialize to the following representation.

{
    'album_name': 'Things We Lost In The Fire',
    'artist': 'Low',
    'tracks': [
        '1: Sunflower',
        '2: Whitetail',
        '3: Dinosaur Act',
        ...
    ]
}

This field is read only.

Arguments:

  • many - If applied to a to-many relationship, you should set this argument to True.

PrimaryKeyRelatedField

PrimaryKeyRelatedField may be used to represent the target of the relationship using its primary key.

For example, the following serializer:

class AlbumSerializer(serializers.ModelSerializer):
    tracks = serializers.PrimaryKeyRelatedField(many=True, read_only=True)

    class Meta:
        model = Album
        fields = ('album_name', 'artist', 'tracks')

Would serialize to a representation like this:

{
    'album_name': 'The Roots',
    'artist': 'Undun',
    'tracks': [
        89,
        90,
        91,
        ...
    ]
}

By default this field is read-write, although you can change this behavior using the read_only flag.

Arguments:

  • many - If applied to a to-many relationship, you should set this argument to True.
  • required - If set to False, the field will accept values of None or the empty-string for nullable relationships.
  • queryset - By default ModelSerializer classes will use the default queryset for the relationship. Serializer classes must either set a queryset explicitly, or set read_only=True.

HyperlinkedRelatedField

HyperlinkedRelatedField may be used to represent the target of the relationship using a hyperlink.

For example, the following serializer:

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')

Would serialize to a representation like this:

{
    '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/',
        ...
    ]
}

By default this field is read-write, although you can change this behavior using the read_only flag.

Arguments:

  • view_name - The view name that should be used as the target of the relationship. If you're using the standard router classes this wil be a string with the format <modelname>-detail. required.
  • many - If applied to a to-many relationship, you should set this argument to True.
  • required - If set to False, the field will accept values of None or the empty-string for nullable relationships.
  • queryset - By default ModelSerializer classes will use the default queryset for the relationship. Serializer classes must either set a queryset explicitly, or set read_only=True.
  • 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'.
  • format - If using format suffixes, hyperlinked fields will use the same format suffix for the target unless overridden by using the format argument.

SlugRelatedField

SlugRelatedField may be used to represent the target of the relationship using a field on the target.

For example, the following serializer:

class AlbumSerializer(serializers.ModelSerializer):
    tracks = serializers.SlugRelatedField(many=True, read_only=True,
                                          slug_field='title')

    class Meta:
        model = Album
        fields = ('album_name', 'artist', 'tracks')

Would serialize to a representation like this:

{
    'album_name': 'Dear John',
    'artist': 'Loney Dear',
    'tracks': [
        'Airport Surroundings',
        'Everything Turns to You',
        'I Was Only Going Out',
        ...
    ]
}

By default this field is read-write, although you can change this behavior using the read_only flag.

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.

Arguments:

  • 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
  • many - If applied to a to-many relationship, you should set this argument to True.
  • required - If set to False, the field will accept values of None or the empty-string for nullable relationships.
  • queryset - By default ModelSerializer classes will use the default queryset for the relationship. Serializer classes must either set a queryset explicitly, or set read_only=True.

HyperlinkedIdentityField

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:

class AlbumSerializer(serializers.HyperlinkedModelSerializer):
    track_listing = serializers.HyperlinkedIdentityField(view_name='track-list')

    class Meta:
        model = Album
        fields = ('album_name', 'artist', 'track_listing')

Would serialize to a representation like this:

{
    'album_name': 'The Eraser',
    'artist': 'Thom Yorke',
    'track_listing': 'http://www.example.com/api/track_list/12/',
}

This field is always read-only.

Arguments:

  • view_name - The view name that should be used as the target of the relationship. If you're using the standard router classes this wil be a string with the format <model_name>-detail. required.
  • 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'.
  • format - If using format suffixes, hyperlinked fields will use the same format suffix for the target unless overridden by using the format argument.

Nested relationships

Nested relationships can be expressed by using serializers as fields.

If the field is used to represent a to-many relationship, you should add the many=True flag to the serializer field.

Example

For example, the following serializer:

class TrackSerializer(serializers.ModelSerializer):
    class Meta:
        model = Track
        fields = ('order', 'title')

class AlbumSerializer(serializers.ModelSerializer):
    tracks = TrackSerializer(many=True)

    class Meta:
        model = Album
        fields = ('album_name', 'artist', 'tracks')

Would serialize to a nested representation like this:

{
    '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'},
        ...
    ],
}

Custom relational fields

To implement a custom relational field, you should override RelatedField, and implement the .to_native(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.

If you want to implement a read-write relational field, you must also implement the .from_native(self, data) method, and add read_only = False to the class definition.

Example

For, example, we could define a relational field, to serialize a track to a custom string representation, using its ordering, title, and duration.

import time

class TrackListingField(serializers.RelatedField):
    def to_native(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')

This custom field would then serialize to the following representation.

{
    '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)',
        ...
    ]
}

Further notes

Reverse relations

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:

class AlbumSerializer(serializers.ModelSerializer):
    class Meta:
        fields = ('tracks', ...)

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:

class Track(models.Model):
    album = models.ForeignKey(Album, related_name='tracks')
    ...

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:

class AlbumSerializer(serializers.ModelSerializer):
    class Meta:
        fields = ('track_set', ...)

See the Django documentation on reverse relationships for more details.

Generic relationships

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.

For example, given the following model for a tag, which has a generic relationship with other arbitrary models:

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

And the following two models, which may be have associated tags:

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)

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.

class TaggedObjectRelatedField(serializers.RelatedField):
    """
    A custom field to use for the `tagged_object` generic relationship.
    """

    def to_native(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')

If you need the target of the relationship to have a nested representation, you can use the required serializers inside the .to_native() method:

    def to_native(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

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.

For more information see the Django documentation on generic relations.

ManyToManyFields with a Through Model

By default, relational fields that target a ManyToManyField with a through model specified are set to read-only.

If you explicitly specify a relational field pointing to a ManyToManyField with a through model, be sure to set read_only to True.

Advanced Hyperlinked fields

If you have very specific requirements for the style of your hyperlinked relationships you can override HyperlinkedRelatedField.

There are two methods you'll need to override.

get_url(self, obj, view_name, request, format)

This method should return the URL that corresponds to the given object.

May raise a NoReverseMatch if the view_name and lookup_field attributes are not configured to correctly match the URL conf.

get_object(self, queryset, view_name, view_args, view_kwargs)

This method should the object that corresponds to the matched URL conf arguments.

May raise an ObjectDoesNotExist exception.

Example

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: 

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)

Deprecated APIs

The following classes have been deprecated, in favor of the many=<bool> syntax. They continue to function, but their usage will raise a PendingDeprecationWarning, which is silent by default.

  • ManyRelatedField
  • ManyPrimaryKeyRelatedField
  • ManyHyperlinkedRelatedField
  • ManySlugRelatedField

The null=<bool> flag has been deprecated in favor of the required=<bool> flag. It will continue to function, but will raise a PendingDeprecationWarning.

In the 2.3 release, these warnings will be escalated to a DeprecationWarning, which is loud by default. In the 2.4 release, these parts of the API will be removed entirely.

For more details see the 2.2 release announcement.

Third Party Packages

The following third party packages are also available.

DRF Nested Routers

The drf-nested-routers package provides routers and relationship fields for working with nested resources.