Deleting the tags that are not used by any other object

Question

I am trying to modify this file https://github.com/alex/django-taggit/blob/master/taggit/models.py so that when a tag is not associated with any other object, it gets deleted.

Here's what I added to the end of the file:

# ---

# Delete the tags that are not used by any other object
from django.db.models.signals import post_delete

def after_deleting(sender, instance, **kwargs):
    if TaggedItem.objects.filter(tag=instance.tag_id).count() == 0:
        print "Deleting tag", instance
        t = Tag.objects.get(pk=instance.tag_id)
        t.delete()

post_delete.connect(after_deleting, sender=TaggedItem)

It's not working as expected. When I run it give this error:

Exception Type: DoesNotExist
Exception Value:    Tag matching query does not exist.

Your help would be appreciated.

Answer 1

I don't think you should be using instance.tag_id in the filter. Try just using instance.tag. Then when finding the tag object you can replace -

t = Tag.objects.get(pk=instance.tag_id)

with -

t = instance.tag

Adding _id to a field is a shortcut for getting the primary key of an object. So instance.tag is the tag object and instance.tag_id is the primary key of the tag object.

The whole thing would be more succinct -

# Delete the tags that are not used by any other object
from django.db.models.signals import post_delete

def after_deleting(sender, instance, **kwargs):
    if not TaggedItem.objects.filter(tag=instance.tag):
        print "Deleting tag", instance
        instance.tag.delete()

post_delete.connect(after_deleting, sender=TaggedItem)