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Docs for related fields, with lots of examples.

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Tom Christie 2013-02-07 23:11:12 +00:00
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docs/api-guide/relations.md
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@ -12,92 +12,155 @@ Relational fields are used to represent model relationships. They can be applie
---
**Note:** The relational fields are declared in `relations.py`, but by convention you should import them using `from rest_framework import serializers` and refer to fields as `serializers.<FieldName>`.
**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>`.
---
## RelatedField
# API Reference
This field can be applied to any of the following:
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.
* A `ForeignKey` field.
* A `OneToOneField` field.
* A reverse OneToOne relationship
* Any other "to-one" relationship.
class Album(models.Model):
album_name = models.CharField(max_length=100)
artist = models.CharField(max_length=100)
By default `RelatedField` will represent the target of the field using it's `__unicode__` method.
You can customize this behavior by subclassing `ManyRelatedField`, and overriding the `.to_native(self, value)` method.
## ManyRelatedField
This field can be applied to any of the following:
* A `ManyToManyField` field.
* A reverse ManyToMany relationship.
* A reverse ForeignKey relationship
* Any other "to-many" relationship.
By default `ManyRelatedField` will represent the targets of the field using their `__unicode__` method.
For example, given the following models:
class TaggedItem(models.Model):
"""
Tags arbitrary model instances using a generic relation.
See: https://docs.djangoproject.com/en/dev/ref/contrib/contenttypes/
"""
tag = models.SlugField()
content_type = models.ForeignKey(ContentType)
object_id = models.PositiveIntegerField()
content_object = GenericForeignKey('content_type', 'object_id')
def __unicode__(self):
return self.tag
class Bookmark(models.Model):
"""
A bookmark consists of a URL, and 0 or more descriptive tags.
"""
url = models.URLField()
tags = GenericRelation(TaggedItem)
And a model serializer defined like this:
class BookmarkSerializer(serializers.ModelSerializer):
tags = serializers.ManyRelatedField()
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:
model = Bookmark
exclude = ('id',)
unique_together = ('album', 'order')
def __unicode__(self):
return '%d: %s' % (self.order, self.title)
Then an example output format for a Bookmark instance would be:
## RelatedField
`RelatedField` may be used to represent the target of the relationship using it's `__unicode__` method.
For example, the following serializer.
class AlbumSerializer(serializer.ModelSerializer):
tracks = RelatedField(many=True)
class Meta:
fields = ('album_name', 'artist', 'tracks')
Would serialize to the following representation.
{
'tags': [u'django', u'python'],
'url': u'https://www.djangoproject.com/'
'album_name': 'Things We Lost In The Fire',
'artist': 'Low'
'tracks': [
'1: Sunflower',
'2: Whitetail',
'3: Dinosaur Act',
...
]
}
This field is read only.
## PrimaryKeyRelatedField
## ManyPrimaryKeyRelatedField
`PrimaryKeyRelatedField` and `ManyPrimaryKeyRelatedField` will represent the target of the relationship using it's primary key.
`PrimaryKeyRelatedField` may be used to represent the target of the relationship using it's primary key.
By default these fields are read-write, although you can change this behavior using the `read_only` flag.
For example, the following serializer:
class AlbumSerializer(serializer.ModelSerializer):
tracks = PrimaryKeyRelatedField(many=True, read_only=True)
class Meta:
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` - 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`.
* `null` - If set to `True`, the field will accept values of `None` or the empty-string for nullable relationships.
* `required` - If set to `False`, the field will accept values of `None` or the empty-string for nullable relationships.
## HyperlinkedRelatedField
`HyperlinkedRelatedField` may be used to represent the target of the relationship using a hyperlink.
For example, the following serializer:
class AlbumSerializer(serializer.ModelSerializer):
tracks = HyperlinkedRelatedField(many=True, read_only=True,
view_name='track-detail')
class Meta:
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. **required**.
* `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`.
* `slug_field` - The field on the target that should be used for the lookup. Default is `'slug'`.
* `pk_url_kwarg` - The named url parameter for the pk field lookup. Default is `pk`.
* `slug_url_kwarg` - The named url parameter for the slug field lookup. Default is to use the same value as given for `slug_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
## ManySlugRelatedField
`SlugRelatedField` and `ManySlugRelatedField` will represent the target of the relationship using a unique slug.
`SlugRelatedField` may be used to represent the target of the relationship using a field on the target.
By default these fields read-write, although you can change this behavior using the `read_only` flag.
For example, the following serializer:
class AlbumSerializer(serializer.ModelSerializer):
tracks = SlugRelatedField(many=True, read_only=True, slug_field='title')
class Meta:
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**:
@ -105,35 +168,181 @@ By default these fields read-write, although you can change this behavior using
* `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`.
* `null` - If set to `True`, the field will accept values of `None` or the empty-string for nullable relationships.
## HyperlinkedRelatedField
## ManyHyperlinkedRelatedField
`HyperlinkedRelatedField` and `ManyHyperlinkedRelatedField` will represent the target of the relationship using a hyperlink.
By default, `HyperlinkedRelatedField` 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. **required**.
* `format` - If using format suffixes, hyperlinked fields will use the same format suffix for the target unless overridden by using the `format` argument.
* `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`.
* `slug_field` - The field on the target that should be used for the lookup. Default is `'slug'`.
* `pk_url_kwarg` - The named url parameter for the pk field lookup. Default is `pk`.
* `slug_url_kwarg` - The named url parameter for the slug field lookup. Default is to use the same value as given for `slug_field`.
* `null` - If set to `True`, the field will accept values of `None` or the empty-string for nullable relationships.
## HyperLinkedIdentityField
This field can be applied as an identity relationship, such as the `'url'` field on a HyperlinkedModelSerializer.
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 = HyperLinkedIdentityField(view_name='track-list')
class Meta:
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. **required**.
* `format` - If using format suffixes, hyperlinked fields will use the same format suffix for the target unless overridden by using the `format` argument.
* `slug_field` - The field on the target that should be used for the lookup. Default is `'slug'`.
* `pk_url_kwarg` - The named url parameter for the pk field lookup. Default is `pk`.
* `slug_url_kwarg` - The named url parameter for the slug field lookup. Default is to use the same value as given for `slug_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. For example:
class TrackSerializer(serializer.ModelSerializer):
class Meta:
fields = ('order', 'title')
class AlbumSerializer(serializer.ModelSerializer):
tracks = TrackSerializer(many=True)
class Meta:
fields = ('album_name', 'artist', 'tracks')
Note that nested relationships are currently read-only. For read-write relationships, you should use a flat relational style.
## 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.
class TrackListingField(serializers.RelatedField):
def to_native(self, value):
return 'Track %d: %s' % (value.ordering, value.name)
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.
# Further notes
## Reverse relations
Note that reverse relationships are not automatically generated by the `ModelSerializer` and `HyperlinkedModelSerializer` classes. To include a reverse relationship, you cannot simply add it to the fields list.
**The following will not work:**
class AlbumSerializer(serializer.ModelSerializer):
class Meta:
fields = ('tracks', ...)
Instead, you must explicitly add it to the serializer. For example:
class AlbumSerializer(serializer.ModelSerializer):
tracks = serializers.PrimaryKeyRelationship(many=True)
...
By default, the field will uses the same accessor as it's field name to retrieve the relationship, so in this example, `Album` instances would need to have the `tracks` attribute for this relationship to work.
The best way to ensure this is typically to make sure that the relationship on the model definition has it's `related_name` argument properly set. For example:
class Track(models.Model):
album = models.ForeignKey(Album, related_name='tracks')
...
Alternatively, you can use the `source` argument on the serializer field, to use a different accessor attribute than the field name. For example.
class AlbumSerializer(serializer.ModelSerializer):
tracks = serializers.PrimaryKeyRelationship(many=True, source='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_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][generic-relations].
---
## Deprecated relational fields
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.
In the 2.3 release, this warning will be escalated to a `DeprecationWarning`.
In the 2.4 release, they will be removed entirely.
* `ManyRelatedField`
* `ManyPrimaryKeyRelatedField`
* `ManyHyperlinkedRelatedField`
* `ManySlugRelatedField`
[cite]: http://lwn.net/Articles/193245/
[reverse-relationships]: https://docs.djangoproject.com/en/dev/topics/db/queries/#following-relationships-backward
[generic-relations]: https://docs.djangoproject.com/en/dev/ref/contrib/contenttypes/#id1