Relax check E042 preventing including stored generated fields in composite primary keys

[David Sanders]

Currently E042 prevents including GeneratedField in a CompositePrimaryKey.

Postgres & MySQL are both fine with including stored generated columns in primary keys, including when foreign keys reference these pks. Postgres (18) & MySQL both reject the inclusion of virtual generated columns.

SQLite doesn't allow generated columns altogether: ​https://www.sqlite.org/gencol.html#limitations

I don't have an Oracle instance to test with.

Django should not get in the way of a valid database operations for vendors that do support a feature. I realise that --skip-checks can be passed in but then we'd be bypassing other useful checks for the lifetime of a codebase.

Personally I believe we should remove the generated columns part of E042 altogether as the user will find out the mistake soon enough when the database complains during migration.

Another option may be to just only check for virtual generated columns.

Forum thread: ​https://forum.djangoproject.com/t/can-we-relax-the-restriction-e042-on-including-generated-fields-in-composite-pks/41438

[Simon Charette]

Accepting on the basis that we should do something here.

Whether we should

1. Remove the check altogether for GeneratedField
2. Remove the check for GeneratedField(db_persist=False)
3. Add a feature flag supports_stored_generated_field_pk and base check of it

is up for debate but given we have a whole section on _Database limitations_ in the GeneratedField docs it seems like 1. is the option that makes the most sense.

[David Sanders]

For 2 I think you mean db_persist=True 😉

But yeah I just noticed that option 1. actually is inline with this:

    pk = GeneratedField(primary_key=True, ...)

There's no check for this but it will work as long as db_persist=True for mysql & pg, but will fail for sqlite.

[Sarah Boyce]

Given #36472, the restriction (while can be harsh) does make some sense and can be relaxed once #36472 is resolved. Hence, I don't plan to backport to 5.2