Fixes: #18717 - On delete signal handling, manually save the related object in a ManyToOneRel to trigger a change record

[bctiemann]

Fixes: #18717

The handle_deleted_object signal handler ensures that change records are saved for the object being deleted, but also manually triggers a m2m_changed signal to ensure that related objects through many-to-many relationships are also change-logged through their own signal handlers. However, we do not handle the case where the related object is linked to the deleted object through a many-to-one relationship.

This change handles ManyToOneRel fields in the same area of code, by resolving the related object in the same way as with M2M fields, but instead setting the related object's field value to None and saving, thus ensuring a change record is created for the related object.

Note the check on L164 which ensures the related field is null=True. This is primarily to ensure that unit tests pass, as without it one of the tests fails due to trying to set a non-nullable field to None. However I think in practice this will not happen as in the case of any FK relationship where null=False, trying to delete an object which is pointed to by a related object via a many-to-one relationship will be caught by object-level validation before the deleted object handler is ever called.

[jnovinger]

Spent some time thinking on this and trying to poke holes in it, but I got nothing.

I think in an ideal world, we'd have a test suite for handle_deleted_object and could add tests to make sure this behaves as expected. But, that seems out of scope for this fix.