GenericRelation uses read connection for writes

[Jake Howard]

When trying to modify a GenericRelation (the other side of a GenericForeignKey) in a multi-DB environment the read connection ("replica") is used:

instance.relation.db  # replica - CORRECT

instance.relation.update(field="value")  # also uses the replica - WRONG

This includes when explicitly requesting the "default" DB:

instance = MyModel.objects.using("default").first()

instance.relation.update(field="value")  # also uses the replica - WRONG

In cases where the db_for_read is read-only (common in HA environments), this produces an error.

I've created a minimal reproduction with a failing test to demo: ​https://github.com/RealOrangeOne/django-generic-relation-db-repro. I've tested this against Django 4.2 - 5.2.

I did some digging, and it seems to be something around the db property. Model.objects.update seems to correctly switch to the write instance, but I can't quite see where. My guess is that the generic relation needs to do the same, but the inheritance and dynamic classes makes it hard to trace.

[Sarah Boyce]

Thank you for the test project!

[Simon Charette]

Slightly related to #15610.