136 lines
4.8 KiB
Python
136 lines
4.8 KiB
Python
"""
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34. Generic relations
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Generic relations let an object have a foreign key to any object through a
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content-type/object-id field. A generic foreign key can point to any object,
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be it animal, vegetable, or mineral.
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The canonical example is tags (although this example implementation is *far*
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from complete).
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"""
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from django.db import models
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from django.contrib.contenttypes.models import ContentType
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from django.contrib.contenttypes import generic
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class TaggedItem(models.Model):
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"""A tag on an item."""
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tag = models.SlugField()
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content_type = models.ForeignKey(ContentType)
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object_id = models.PositiveIntegerField()
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content_object = generic.GenericForeignKey()
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class Meta:
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ordering = ["tag"]
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def __str__(self):
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return self.tag
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class Animal(models.Model):
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common_name = models.CharField(maxlength=150)
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latin_name = models.CharField(maxlength=150)
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tags = generic.GenericRelation(TaggedItem)
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def __str__(self):
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return self.common_name
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class Vegetable(models.Model):
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name = models.CharField(maxlength=150)
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is_yucky = models.BooleanField(default=True)
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tags = generic.GenericRelation(TaggedItem)
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def __str__(self):
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return self.name
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class Mineral(models.Model):
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name = models.CharField(maxlength=150)
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hardness = models.PositiveSmallIntegerField()
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# note the lack of an explicit GenericRelation here...
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def __str__(self):
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return self.name
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__test__ = {'API_TESTS':"""
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# Create the world in 7 lines of code...
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>>> lion = Animal(common_name="Lion", latin_name="Panthera leo")
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>>> platypus = Animal(common_name="Platypus", latin_name="Ornithorhynchus anatinus")
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>>> eggplant = Vegetable(name="Eggplant", is_yucky=True)
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>>> bacon = Vegetable(name="Bacon", is_yucky=False)
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>>> quartz = Mineral(name="Quartz", hardness=7)
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>>> for o in (lion, platypus, eggplant, bacon, quartz):
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... o.save()
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# Objects with declared GenericRelations can be tagged directly -- the API
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# mimics the many-to-many API.
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>>> bacon.tags.create(tag="fatty")
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<TaggedItem: fatty>
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>>> bacon.tags.create(tag="salty")
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<TaggedItem: salty>
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>>> lion.tags.create(tag="yellow")
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<TaggedItem: yellow>
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>>> lion.tags.create(tag="hairy")
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<TaggedItem: hairy>
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>>> lion.tags.all()
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[<TaggedItem: hairy>, <TaggedItem: yellow>]
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>>> bacon.tags.all()
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[<TaggedItem: fatty>, <TaggedItem: salty>]
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# You can easily access the content object like a foreign key.
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>>> t = TaggedItem.objects.get(tag="salty")
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>>> t.content_object
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<Vegetable: Bacon>
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# Recall that the Mineral class doesn't have an explicit GenericRelation
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# defined. That's OK, because you can create TaggedItems explicitly.
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>>> tag1 = TaggedItem(content_object=quartz, tag="shiny")
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>>> tag2 = TaggedItem(content_object=quartz, tag="clearish")
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>>> tag1.save()
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>>> tag2.save()
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# However, excluding GenericRelations means your lookups have to be a bit more
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# explicit.
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>>> from django.contrib.contenttypes.models import ContentType
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>>> ctype = ContentType.objects.get_for_model(quartz)
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>>> TaggedItem.objects.filter(content_type__pk=ctype.id, object_id=quartz.id)
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[<TaggedItem: clearish>, <TaggedItem: shiny>]
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# You can set a generic foreign key in the way you'd expect.
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>>> tag1.content_object = platypus
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>>> tag1.save()
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>>> platypus.tags.all()
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[<TaggedItem: shiny>]
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>>> TaggedItem.objects.filter(content_type__pk=ctype.id, object_id=quartz.id)
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[<TaggedItem: clearish>]
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# If you delete an object with an explicit Generic relation, the related
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# objects are deleted when the source object is deleted.
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# Original list of tags:
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>>> [(t.tag, t.content_type, t.object_id) for t in TaggedItem.objects.all()]
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[('clearish', <ContentType: mineral>, 1), ('fatty', <ContentType: vegetable>, 2), ('hairy', <ContentType: animal>, 1), ('salty', <ContentType: vegetable>, 2), ('shiny', <ContentType: animal>, 2), ('yellow', <ContentType: animal>, 1)]
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>>> lion.delete()
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>>> [(t.tag, t.content_type, t.object_id) for t in TaggedItem.objects.all()]
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[('clearish', <ContentType: mineral>, 1), ('fatty', <ContentType: vegetable>, 2), ('salty', <ContentType: vegetable>, 2), ('shiny', <ContentType: animal>, 2)]
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# If Generic Relation is not explicitly defined, any related objects
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# remain after deletion of the source object.
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>>> quartz.delete()
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>>> [(t.tag, t.content_type, t.object_id) for t in TaggedItem.objects.all()]
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[('clearish', <ContentType: mineral>, 1), ('fatty', <ContentType: vegetable>, 2), ('salty', <ContentType: vegetable>, 2), ('shiny', <ContentType: animal>, 2)]
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# If you delete a tag, the objects using the tag are unaffected
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# (other than losing a tag)
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>>> tag = TaggedItem.objects.get(id=1)
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>>> tag.delete()
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>>> bacon.tags.all()
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[<TaggedItem: salty>]
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>>> [(t.tag, t.content_type, t.object_id) for t in TaggedItem.objects.all()]
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[('clearish', <ContentType: mineral>, 1), ('salty', <ContentType: vegetable>, 2), ('shiny', <ContentType: animal>, 2)]
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"""}
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