Possible bug in prefetch_related used with Reverse generic relations

[Todor Velichkov]

Hi, guys.
About a week ago i asked the same question on stackoverflow. Since i got no response there i started to read how to report a bug (because I've never done it before), and this article took me here ;). I need more advanced opinion if this is truly a bug which should be reported or i just want too much from the ORM. So i will repeat myself from stackoverflow and explain what is all about. 

Here is some example model structure:

from django.db import models
from django.contrib.contenttypes.fields import GenericForeignKey, GenericRelation
from django.contrib.contenttypes.models import ContentType


class TaggedItem(models.Model):
 tag = models.SlugField()
 content_type = models.ForeignKey(ContentType)
 object_id = models.PositiveIntegerField()
 content_object = GenericForeignKey('content_type', 'object_id')

 def __unicode__(self):
 return self.tag

class Movie(models.Model):
 name = models.CharField(max_length=100)

 def __unicode__(self):
 return self.name


class Author(models.Model):
 name = models.CharField(max_length=100)

 def __unicode__(self):
 return self.name


class Book(models.Model):
 name = models.CharField(max_length=100)
 author = models.ForeignKey(Author)
 tags = GenericRelation(TaggedItem, related_query_name='books')

 def __unicode__(self):
 return self.name

Now, lets create some initial data:

>>> from tags.models import Book, Movie, Author, TaggedItem
>>> a = Author(name='E L James')
>>> a.save()
>>> b1 = Book(name='Fifty Shades of Grey', author=a)
>>> b1.save()
>>> b2 = Book(name='Fifty Shades Darker', author=a)
>>> b2.save()
>>> b3 = Book(name='Fifty Shades Freed', author=a)
>>> b3.save()
>>> t1 = TaggedItem(content_object=b1, tag='roman')
>>> t1.save()
>>> t2 = TaggedItem(content_object=b2, tag='roman')
>>> t2.save()
>>> t3 = TaggedItem(content_object=b3, tag='roman')
>>> t3.save()
>>> m1 = Movie(name='Guardians of the Galaxy')
>>> m1.save()
>>> t4 = TaggedItem(content_object=m1, tag='action movie')
>>> t4.save()

Now we can make a query like this:

>>> TaggedItem.objects.filter(books__author__name='E L James')
[<TaggedItem: roman>, <TaggedItem: roman>, <TaggedItem: roman>]

But we cant do this:

>>> TaggedItem.objects.filter(books__author__name='E L James').prefetch_related('books')
Traceback (most recent call last):
...
AttributeError: 'Book' object has no attribute 'object_id'

Why i think this may be a bug? 
Because of the error. Normally if prefech_related can't resolve a relation it complains like that:

AttributeError: Cannot find 'some_field' on TaggedItem object, 'some_field' is an invalid parameter to prefetch_related()

But in the example above, Django actually tries to resolve the relation and fails because the ORM is searching for object_id inside the Book model instead of the TaggedItem model. 

Why i don't use the forward relation (the `content_type` field).
Yes, this actually will work:

>>> TaggedItem.objects.filter(books__author__name='E L James').prefetch_related('content_object')
[<TaggedItem: roman>, <TaggedItem: roman>, <TaggedItem: roman>]

 

But this way u can't go further with the relations against a queryset containing different types of content types.

>>> TaggedItem.objects.all().prefetch_related('content_object__author')
Traceback (most recent call last):
  ...
AttributeError: 'Movie' object has no attribute 'author_id'

 While the reversed API kind of a give me hope that it's the right way to go. I mean the ORM don't need to guess the author of which content types need  to be prefetched.

TaggedItem.objects.all().prefetch_related('books__author')

We clearly show that we want the authors of all book models. nothing more, nothing less.