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		"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
}}}"	New feature	new	Database layer (models, ORM)	dev	Normal		uuid autofield	Diego Lima Brian Helba raydeal johnthagen Michael Kuron Alireza Safari Petr Přikryl Wilson E. Husin Luiz Braga Dmytro Litvinov Olivier Dalang Ülgen Sarıkavak Simon Charette Nick Pope Tomasz Wójcik Ryan P Kilby	Accepted	1	0	0	1	0	0