Possible bug in prefetch_related used with Reverse generic relations

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.

 

--
You received this message because you are subscribed to the Google Groups "Django users" group.
To unsubscribe from this group and stop receiving emails from it, send an email to django-users+unsubscribe@googlegroups.com.
To post to this group, send email to django-users@googlegroups.com.
Visit this group at http://groups.google.com/group/django-users.
To view this discussion on the web visit https://groups.google.com/d/msgid/django-users/557ff1af-41d2-4676-90b9-cb19b2478730%40googlegroups.com.
For more options, visit https://groups.google.com/d/optout.