> 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>`.
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.
*`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`.
*`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
`SlugRelatedField` may be used to represent the target of the relationship using a field on the target.
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`.
*`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`.
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):
*`format` - If using format suffixes, hyperlinked fields will use the same format suffix for the target unless overridden by using the `format` argument.
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 it's ordering, title, and duration.
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.
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.
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.
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].
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][2.2-announcement].
