109 lines
3.3 KiB
Python
109 lines
3.3 KiB
Python
"""
|
|
33. Generic relations
|
|
|
|
Generic relations let an object have a foreign key to any object through a
|
|
content-type/object-id field. A generic foreign key can point to any object,
|
|
be it animal, vegetable, or mineral.
|
|
|
|
The canonical example is tags (although this example implementation is *far*
|
|
from complete).
|
|
"""
|
|
|
|
from django.db import models
|
|
from django.contrib.contenttypes.models import ContentType
|
|
|
|
class TaggedItem(models.Model):
|
|
"""A tag on an item."""
|
|
tag = models.SlugField()
|
|
content_type = models.ForeignKey(ContentType)
|
|
object_id = models.PositiveIntegerField()
|
|
|
|
content_object = models.GenericForeignKey()
|
|
|
|
class Meta:
|
|
ordering = ["tag"]
|
|
|
|
def __str__(self):
|
|
return self.tag
|
|
|
|
class Animal(models.Model):
|
|
common_name = models.CharField(maxlength=150)
|
|
latin_name = models.CharField(maxlength=150)
|
|
|
|
tags = models.GenericRelation(TaggedItem)
|
|
|
|
def __str__(self):
|
|
return self.common_name
|
|
|
|
class Vegetable(models.Model):
|
|
name = models.CharField(maxlength=150)
|
|
is_yucky = models.BooleanField(default=True)
|
|
|
|
tags = models.GenericRelation(TaggedItem)
|
|
|
|
def __str__(self):
|
|
return self.name
|
|
|
|
class Mineral(models.Model):
|
|
name = models.CharField(maxlength=150)
|
|
hardness = models.PositiveSmallIntegerField()
|
|
|
|
# note the lack of an explicit GenericRelation here...
|
|
|
|
def __str__(self):
|
|
return self.name
|
|
|
|
API_TESTS = """
|
|
# Create the world in 7 lines of code...
|
|
>>> lion = Animal(common_name="Lion", latin_name="Panthera leo")
|
|
>>> platypus = Animal(common_name="Platypus", latin_name="Ornithorhynchus anatinus")
|
|
>>> eggplant = Vegetable(name="Eggplant", is_yucky=True)
|
|
>>> bacon = Vegetable(name="Bacon", is_yucky=False)
|
|
>>> quartz = Mineral(name="Quartz", hardness=7)
|
|
>>> for o in (lion, platypus, eggplant, bacon, quartz):
|
|
... o.save()
|
|
|
|
# Objects with declared GenericRelations can be tagged directly -- the API
|
|
# mimics the many-to-many API.
|
|
>>> lion.tags.create(tag="yellow")
|
|
<TaggedItem: yellow>
|
|
>>> lion.tags.create(tag="hairy")
|
|
<TaggedItem: hairy>
|
|
>>> bacon.tags.create(tag="fatty")
|
|
<TaggedItem: fatty>
|
|
>>> bacon.tags.create(tag="salty")
|
|
<TaggedItem: salty>
|
|
|
|
>>> lion.tags.all()
|
|
[<TaggedItem: hairy>, <TaggedItem: yellow>]
|
|
>>> bacon.tags.all()
|
|
[<TaggedItem: fatty>, <TaggedItem: salty>]
|
|
|
|
# You can easily access the content object like a foreign key.
|
|
>>> t = TaggedItem.objects.get(tag="salty")
|
|
>>> t.content_object
|
|
<Vegetable: Bacon>
|
|
|
|
# Recall that the Mineral class doesn't have an explicit GenericRelation
|
|
# defined. That's OK, because you can create TaggedItems explicitly.
|
|
>>> tag1 = TaggedItem(content_object=quartz, tag="shiny")
|
|
>>> tag2 = TaggedItem(content_object=quartz, tag="clearish")
|
|
>>> tag1.save()
|
|
>>> tag2.save()
|
|
|
|
# However, excluding GenericRelations means your lookups have to be a bit more
|
|
# explicit.
|
|
>>> from django.contrib.contenttypes.models import ContentType
|
|
>>> ctype = ContentType.objects.get_for_model(quartz)
|
|
>>> TaggedItem.objects.filter(content_type__pk=ctype.id, object_id=quartz.id)
|
|
[<TaggedItem: clearish>, <TaggedItem: shiny>]
|
|
|
|
# You can set a generic foreign key in the way you'd expect.
|
|
>>> tag1.content_object = platypus
|
|
>>> tag1.save()
|
|
>>> platypus.tags.all()
|
|
[<TaggedItem: shiny>]
|
|
>>> TaggedItem.objects.filter(content_type__pk=ctype.id, object_id=quartz.id)
|
|
[<TaggedItem: clearish>]
|
|
"""
|