I wonder how this will play with existing SQLite tables that don't have DEFERRABLE INITIALLY DEFERRED constraints. At this point I think the best course of action would be to mention it in the release notes.

I'm not sure exactly what the ramifications might be. Anything specific to mention besides what I wrote so far?

I think the text you added is appropriate. Makes me wonder if we would like to document how to migrate existing foreign keys to DEFERRABLE INITIALLY DEFERRED (which will be a bit complicated as it will require a table rebuild since SQLite doesn't support ALTER TABLE) or suggest PRAGMA defer_foreign_keys = ON should be issued on connection creation when entering a transaction for SQLite databases created with Django < 2.0.

Related to what charettes asked, but asked in a slightly different way:

Assumption: basic migrations will handle the table rebuild, like existing sqlite schema changes.

Question: if PRAGMA foreign_keys is going to be default on in django 2, how will the user "opt in" their old tables to a migration, without there being a difference in their model params?

It's probably feasible to create a script or management command or similar to rebuild tables. I wasn't sure if it's needed; i.e. what circumstances will create a problem with non-upgraded tables? The problems in the test suite related to that seemed to be with flush which doesn't like a huge concern (if someone wants to do that, just delete the tables instead).

Something along these lines would break on tables that haven't been rebuilt and still have IMMEDIATE foreign key constraints:

with transaction.atomic():
    # The following line will immediately fail now that foreign key
    # constraints are enforced and were created as IMMEDIATE
    # instead of INITIALLY DEFERRED.
    Book.objects.create(author_id=1)
    Author.objects.create(id=1)

We might want to mention that constraints used to be built as DEFERRABLE IMMEDIATE in the Foreign keys are now created with DEFERRABLE INITIALLY DEFERRED section and might require a rebuild to allow the above pattern to be used.

Thanks, I incorporate this into the release notes.