In our model, we have a Story class, with a boolean 'deleted'
attribute. We defined a custom default manager to return all of the
stories that are not deleted, so that most of the code on the site will
not have to deal with filtering out the deleted stories (there's another
custom manager called with_deleted for those code paths that need to see
the deleted stories as well).
The problem comes up when deleting a user. The deletion code iterates
the user's stories to delete them, but uses the default manager, so
stories with deleted=1 are not found. So (ironically) any stories
marked with the 'deleted' field are not actually deleted when the user
is removed. This prevents the user from being deleted, because the
story table still has a foreign key reference to the user table, and
MySQL prevents the user record from being deleted.
The docs for custom managers say: "it's generally a good idea for the
first Manager to be relatively unfiltered". But it seems that any
filtering in the default manager will interfere with cascading deletes.
If the default manager does any filtering at all, then deleting an object may fail because its related objects will not all be deleted, and the database's referential integrity will prevent the deletion. When looking for related obejcts to delete, you have to always get all of them. No filtering allowed.
