id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
32577	Add support for `UUIDAutoField` `DEFAULT_AUTO_FIELD`	Tomasz Wójcik	nobody	"Default auto field introduced in #31007, [https://docs.djangoproject.com/en/3.2/releases/3.2/#customizing-type-of-auto-created-primary-keys as seen in the docs], is  `BigAutoField`.

{{{

class BigAutoField(AutoFieldMixin, BigIntegerField):
    def get_internal_type(self):
        return 'BigAutoField'

    def rel_db_type(self, connection):
        return BigIntegerField().db_type(connection=connection)

}}}

Many projects prefer UUID field over big int for pk. So far we had to use a mixin for that but I am under the impression `DEFAULT_AUTO_FIELD` exists to lift this burden.

I'd expect something like this to work (additional adjustments might be needed)

{{{

from django.db import models
from django.db.models.fields import AutoFieldMixin


class UUIDAutoField(AutoFieldMixin, models.UUIDField):
    def get_internal_type(self):
        return 'UUIDAutoField'

    def rel_db_type(self, connection):
        return models.UUIDField().db_type(connection=connection)
}}}

But if I use this for `DEFAULT_AUTO_FIELD` I get the error

{{{
ValueError: Primary key 'common.fields.UUIDAutoField' referred by DEFAULT_AUTO_FIELD must subclass AutoField.
}}}

I noticed two things I want to share.

First, the default auto field `BigAutoField` is not subclassing `AutoField` so it's not consistent with the error message.

Second, `AutoField` is subclassing `IntegerField`. If I understand this correctly, users are limited to subclass `IntegerField` for `DEFAULT_AUTO_FIELD`. If so, there's no way to implement `UUIDAutoField` as `DEFAULT_AUTO_FIELD`.

I understand auto fields need to share a common interface but having to subclass `IntegerField` is a big constraint.

I'm happy to open a PR once I know what's the desired functionality. Possible solutions I see:
- enforce subclassing of `AutoFieldMixin` instead `AutoField`
- I don't think this field has to be very generic because DBs pk types are very limited. As far as I know, only ints and UUIDs make sense for pk. Maybe adding `UUIDAutoField` to django fields would make sense. That way it'd have to enforce subclass of `AutoField` or `UUIDAutoField`

Mentioned also in [https://groups.google.com/g/django-developers/c/MBPEPKlibGQ/m/uJVZbMpqBAAJ here], [https://groups.google.com/g/django-developers/c/MBPEPKlibGQ/m/ntEoLCCMBAAJ here] and [https://github.com/django/django/pull/13179#issuecomment-742473840 here].

{{{
Django==3.2rc1
}}}"	Uncategorized	new	Database layer (models, ORM)	3.2	Normal				Unreviewed	0	0	0	0	0	0