N+1 queries on queryset.delete()

[todorvelichkov]

I just found out that we have a N+1 queries on one of our purge operations. I've traced the problem and found out its caused by #523 which is trying to fix issue #521

IMHO a much better approach to the problem (CASCADE DELETE over GFK) would be to implement a GenericRelation in the model definition. This way Django will do it automatically a much more optimized way with a limited number of queries.

From the docs:

tags = GenericRelation(
    TaggedItem,
    content_type_field="content_type_fk",
    object_id_field="object_primary_key",
)

Note also, that if you delete an object that has a GenericRelation, any objects which have a GenericForeignKey pointing at it will be deleted as well. In the example above, this means that if a Bookmark object were deleted, any TaggedItem objects pointing at it would be deleted at the same time.

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.

[auvipy]

would you mind coming with a working solution as PR?

[todorvelichkov]

@auvipy the GenericRelation field is something that everyone should add into the field definition of the models in their project, it should look like this:

class MyModelName(models.Model):
    # .... field definitions
    follow_set = GenericRelation(
        "actstream.follow",
        content_type_field="content_type_id",
        object_id_field="object_id",
        related_query_name="mymodelname",
    )

So in terms of changes, what's necessary is probably the ability to disable the current delete functionality and add documentation for how to handle this with GenericRelations (i.e. the example above).