Efficiently delete orphaned m2m objects/tags in Django

Question

I have two models - Photo and Tag - which are connected via a ManyToManyField.

class Photo(models.Model):
    tags = models.ManyToManyField(Tag)

class Tag(m         


        

Answer 1

We've had to further improve performance of this task, so I modified okm's solution a bit:

    all_tag_pks = Tag.objects.values_list('pk', flat=True)
    used_tag_pks = Photo.tags.through.objects.values_list('tag', flat=True)
    Tag.objects.filter(pk__in=list(set(all_tag_pks) - set(used_tag_pks))).delete()

By that, the query to the database gets a lot smaller and faster.