How can I delete the parent object related through a many to many field?

[Mark Phillips]

I have three related models - Document, Collection, and CollectionDocument. A Collection object is made up of a group of documents.

class Document(Model):
    document_id = models.AutoField(primary_key=True)

class Collection(Model):
    collection_id = models.AutoField(primary_key=True)
    document = models.ManyToManyField(Document, through='CollectionDocument', related_name='collections',) 

class CollectionDocument(Model):
    collection_id = models.ForeignKey(Collection, on_delete=models.CASCADE, )
    document = models.ForeignKey(Document, on_delete=models.CASCADE, )

In the Djangop Admin, I have a DocumentAdmin and a CollectionAdmin with the CollectionDocument as inlines in the CollectionAdmin.

When I delete all the documents associated with a Collection, I also want the Collection to be deleted. Right now, when I delete the documents in a Collection, the CollectioDocument is empty, but the Collection still exists without any documents associated with it.

I have tried several ways to do it using pre- and post-delete signals for the CollectionDocument model, but nothing seems to work. Have I structured my models incorrectly? How do I get the Collection to be deleted when all the associated Documents are deleted?

Thanks!

Mark