Re: Annotated queryset + Prefetch object

Hello Jose,

I wouldn't be surprised if this was caused by a Django bug with prefetches doing

weird things when the same model is referenced through different relations[0].

Simon

[0] https://code.djangoproject.com/ticket/26318

Le samedi 4 mars 2017 13:36:20 UTC-5, Jose Kilo a écrit :

Hi all,

I'm trying to use an annotated queryset inside a Prefetch object. For some reason I'm not getting the expected result.
This is a simplified version of my models and query.

class User(models.Model):

    following = models.ManyToManyField('User', related_name='followers', through='Follow')


class Follow(models.Model):

    following = models.ForeignKey(User, on_delete=models.CASCADE, related_name='_followed')
    followed = models.ForeignKey(User, on_delete=models.CASCADE, related_name='_following')


def test():

    User.objects.all().delete()

    user_1 = User.objects.create()
    user_2 = User.objects.create()

    Follow.objects.create(following=user_1, followed=user_2)

    queryset = (
        User.objects.all()
        .prefetch_related(
            Prefetch(
                'followers', to_attr='prefetched_annotated_followers',
                queryset=(User.objects.all().annotate(
                    followers_count=Count('followers', distinct=True),
                ))
            ),
            Prefetch(
                'followers', to_attr='prefetched_followers',
                queryset=User.objects.all()
            ),
        )
    )

    user = queryset.last()
    print(list(user.followers.all()))            # [<User: User object>]
    print(user.prefetched_followers)             # [<User: User object>]
    print(user.prefetched_annotated_followers)   # []

    return queryset

Why the last result is empty ?

Just in case, I added 'prefetched_followers' to compare both results. If I remove it, 'prefetched_annotated_followers' still doesn't get anything.

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