Bug with prefetch_related

Posted this few weeks ago to the mailinglist, but seems that it’s dead so re-posting here. (Maybe it would be a good idea to retire the mailinglist or post a notification there that it’s not been actively followed?)

So, I have a following model with tags:

tags = TaggableManager(through='knuckles.UUIDTaggedItem', blank=True)

where UUIDTaggedItem is:

class UUIDTaggedItem(GenericUUIDTaggedItemBase, TaggedItemBase):

    class Meta:
        verbose_name = "Tag"
        verbose_name_plural = "Tags"

When I query for the tags I get following:

In [18]: e = LiveEvent.objects.get(slug='dj-tony-004')
In [19]: e.tags.all()

Out[19]: <QuerySet [<Tag: 90s>, <Tag: dj tony>, <Tag: house>, <Tag: radioshow>, <Tag: 00's>, <Tag: trance>, <Tag: 90's>, <Tag: eurodance>]>


In [20]: e = LiveEvent.objects.prefetch_related('tags').get(slug='dj-tony-004')
In [21]: e.tags.all()

Out[21]: <QuerySet []>

So when using prefetch_related, I always get empty tags. Not sure if I’m doing something wrong here or if this is a bug?

I’m using Django version 1.11 and django-taggit version 0.22.1

[Uninen]

Thanks a lot for the patch and the tests, @fumuumuf ! 👍

[fumuumuf]

This occurs because instance_attr value isn’t match _prefetch_related_val. It worked fine by modifying as below. (only MySQL, SQLite3)

https://github.com/jazzband/django-taggit/blob/954d8360102fb2102db246bde1336a7e58ff10f2/taggit/managers.py#L114

            if issubclass(self.through, GenericUUIDTaggedItemBase):
                instance_attr = lambda obj: obj._get_pk_val().hex
            else:
                instance_attr = lambda obj: obj._get_pk_val()

            return (
                qs,
                attrgetter("_prefetch_related_val"),
                instance_attr,
                False,
                self.prefetch_cache_name,
                False,
            )