Efficiently delete orphaned m2m objects/tags in Django

django, many-to-many, orphan, orphaned-objects

Solution

Try sub-query w/ intermediate table

qs = Tag.objects.exclude(pk__in=Book.tags.through.objects.values('tag'))

# then you could
qs.delete()

# or if you need to trigger signal per item
for x in qs:
    x.delete()

Problem

I have two models - Photo and Tag - which are connected via a ManyToManyField. ``` class Photo(models.Model): tags = models.ManyToManyField(Tag) class Tag(models.Model): lang = models.CharField(max_length=2) name_es = models.CharField(max_length=40) name_en = models.CharField(max_length=40) ``` Every once in a while, we get orphaned tags, that are not referenced any more by any photo. Is there an efficient way of deleting those tags? I know about this answer: Django: delete M2M orphan entries? And our solution looks like this at the moment: ``` for tag in Tag.objects.all(): if not tag.photo_set.select_related(): tag.delete() ``` However, with increasing database, the runtime of this script is becoming distressingly high :-P Is there an efficient way of getting a list of all tag IDs from the tags table and then a list of all tag IDs from the many-to-many table to create an intersection list?

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