#33008: prefetch_related for GenericForeignKey - Object doesn't exist behaviour
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Reporter: Martin | Owner: nobody
Svoboda |
Type: | Status: new
Uncategorized |
Component: Database | Version: 3.2
layer (models, ORM) |
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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prefetch_related called for GenericForeignKey sets content_type_id and
object_id to None, if the foreign object doesn't exist. This behaviour is
not documented.
GenericForignKey is often used for audit records, so it can keep links to
non-existing objects. Probably prefetch_related shouldn't touch original
values of object_id and content_type_id and only set content_object to
None.
{{{
from django.contrib.auth.models import User
from django.contrib.contenttypes.fields import GenericForeignKey
from django.contrib.contenttypes.models import ContentType
from django.db import models
class TaggedItem(models.Model):
tag = models.SlugField()
content_type = models.ForeignKey(ContentType,
on_delete=models.CASCADE)
object_id = models.PositiveIntegerField()
content_object = GenericForeignKey('content_type', 'object_id')
# init data
guido = User.objects.create(username='Guido')
t = TaggedItem(content_object=guido, tag='test')
t.save()
guido.delete()
# get content_object normally
tags_1 = TaggedItem.objects.filter(tag='test')
tags_1[0].content_object # returns None
tags_1[0].object_id # returns 1
tags_1[0].content_type_id # returns X
# use prefetch_related
tags_2 =
TaggedItem.objects.filter(tag='test').prefetch_related("content_object")
tags_2[0].content_object # returns None
tags_2[0].object_id # returns None
tags_2[0].content_type_id # returns None
}}}
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Ticket URL: <https://code.djangoproject.com/ticket/33008>
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