Re: [Django] #33008: prefetch_related for GenericForeignKey - Object doesn't exist behaviour

[Django]

#33008: prefetch_related for GenericForeignKey - Object doesn't exist behaviour
--+
 Reporter:  Martin Svoboda|Owner:  nobody
 Type:  Cleanup/optimization  |   Status:  new
Component:  contrib.contenttypes  |  Version:  3.2
 Severity:  Normal|   Resolution:
 Keywords:  Documentation | Triage Stage:  Accepted
Has patch:  0 |  Needs documentation:  0
  Needs tests:  0 |  Patch needs improvement:  0
Easy pickings:  0 |UI/UX:  0
--+
Changes (by Carlton Gibson):

 * cc: Simon Charette, Mariusz Felisiak (added)
 * keywords:   => Documentation
 * type:  Uncategorized => Cleanup/optimization
 * component:  Database layer (models, ORM) => contrib.contenttypes
 * stage:  Unreviewed => Accepted


Comment:

 Hi Martin. I do agree that's a little surprising. I'll accept as a
 Documentation issue on the content types app, but I'll cc Simon and
 Mariusz, in case they want to take it as a bug. Thanks.

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Ticket URL: 
Django 
The Web framework for perfectionists with deadlines.

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[Django] #33008: prefetch_related for GenericForeignKey - Object doesn't exist behaviour

[Django]

#33008: prefetch_related for GenericForeignKey - Object doesn't exist behaviour
-+-
   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  |
-+-
 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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