Cascade delete of model with GenericForeignKey without using GenericRelation

Question

I'm creating a reusable django app which includes a model with GenericForeignKey which I need to be cascade deleted.

This model may be attached to any other. I have no control over target model class as it is outside of the app. This means I can not add GenericRelation field to it and can't force user to add it as target might be in another third-party app.

Assuming we have such models (having NO control over Post and PostGroup):

class Tag(models.Model):
    content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    object_id = models.PositiveIntegerField()
    object = GenericForeignKey()

class PostGroup(models.Model):
    title = models.CharField(max_length=255)

class Post(models.Model):
    title = models.CharField(max_length=255)
    group = models.ForeignKey(PostGroup, on_delete=models.CASCADE)

Is there a way to delete Tag in case of PostGroup queryset is being deleted?

E.g. not only post_group.delete() but also PostGroup.objects.delete().

Answer 1

You can use pre_delete signal to achieve this:

from django.db.models.signals import pre_delete
from django.dispatch import receiver


@receiver(pre_delete) # add relevant sender to signal (not mandatory)
def post_group_deleted(sender, instance, using, **kwargs):
    # Query tags with the instance of PostGroup and delete them
    if isinstance(instance, Tag):
        return

    Tag.objects.filter(
        content_type=ContentType.objects.get_for_model(instance),
        object_id=instance.pk
    ).delete()

See documentation here

Unlike ForeignKey, GenericForeignKey does not accept an on_delete argument to customize this behavior; if desired, you can avoid the cascade-deletion by not using GenericRelation, and alternate behavior can be provided via the pre_delete signal.