mirror of
https://github.com/encode/django-rest-framework.git
synced 2024-11-14 21:56:59 +03:00
479 lines
19 KiB
Markdown
479 lines
19 KiB
Markdown
source: relations.py
|
|
|
|
---
|
|
|
|
**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.
|
|
|
|
---
|
|
|
|
# Serializer relations
|
|
|
|
> Bad programmers worry about the code.
|
|
> Good programmers worry about data structures and their relationships.
|
|
>
|
|
> — [Linus Torvalds][cite]
|
|
|
|
|
|
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>`.
|
|
|
|
---
|
|
|
|
#### Inspecting automatically generated relationships.
|
|
|
|
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.
|
|
|
|
To do so, open the Django shell, using `python manage.py shell`, then import the serializer class, instantiate it, and print the object representation…
|
|
|
|
>>> 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())
|
|
|
|
# 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)
|
|
|
|
## StringRelatedField
|
|
|
|
`StringRelatedField` 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.StringRelatedField(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**:
|
|
|
|
* `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`.
|
|
* `many` - If applied to a to-many relationship, you should set this argument to `True`.
|
|
* `allow_null` - If set to `True`, the field will accept values of `None` or the empty string for nullable relationships. Defaults to `False`.
|
|
|
|
## 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][routers] this will be a string with the format `<modelname>-detail`. **required**.
|
|
* `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`.
|
|
* `many` - If applied to a to-many relationship, you should set this argument to `True`.
|
|
* `allow_null` - If set to `True`, the field will accept values of `None` or the empty string for nullable relationships. Defaults to `False`.
|
|
* `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'`.
|
|
* `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`.
|
|
* `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**
|
|
* `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`.
|
|
* `many` - If applied to a to-many relationship, you should set this argument to `True`.
|
|
* `allow_null` - If set to `True`, the field will accept values of `None` or the empty string for nullable relationships. Defaults to `False`.
|
|
|
|
## 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][routers] this will 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'`.
|
|
* `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`.
|
|
* `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, read_only=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_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.
|
|
|
|
If you want to implement a read-write relational field, you must also implement the `.to_internal_value(self, data)` method.
|
|
|
|
## 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_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')
|
|
|
|
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
|
|
|
|
## The `queryset` argument
|
|
|
|
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.
|
|
|
|
In version 2.x a serializer class could *sometimes* automatically determine the `queryset` argument *if* a `ModelSerializer` class was being used.
|
|
|
|
This behavior is now replaced with *always* using an explicit `queryset` argument for writable relational fields.
|
|
|
|
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.
|
|
|
|
## 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][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_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')
|
|
|
|
If you need the target of the relationship to have a nested representation, you can use the required serializers inside the `.to_representation()` method:
|
|
|
|
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
|
|
|
|
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][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)
|
|
|
|
---
|
|
|
|
# Third Party Packages
|
|
|
|
The following third party packages are also available.
|
|
|
|
## DRF Nested Routers
|
|
|
|
The [drf-nested-routers package][drf-nested-routers] provides routers and relationship fields for working with nested resources.
|
|
|
|
[cite]: http://lwn.net/Articles/193245/
|
|
[reverse-relationships]: https://docs.djangoproject.com/en/dev/topics/db/queries/#following-relationships-backward
|
|
[routers]: http://www.django-rest-framework.org/api-guide/routers#defaultrouter
|
|
[generic-relations]: https://docs.djangoproject.com/en/dev/ref/contrib/contenttypes/#id1
|
|
[2.2-announcement]: ../topics/2.2-announcement.md
|
|
[drf-nested-routers]: https://github.com/alanjds/drf-nested-routers
|