Django 4.2, is a 2nd autofield-like field on a model possible?

Using Django Using the ORM
4

@charettes thank you - that was the breadcrumb I needed. The following rough implementation works:

class Default(Expression):
    def as_sql(self, compiler, connection):
        return "DEFAULT", []


class AutoPositiveBigIntegerField(models.PositiveBigIntegerField):
    @property
    def db_returning(self):
        return True

    def get_default(self):
        return Default()


class Invoice(models.Model):
    ...
    number = AutoPositiveBigIntegerField(unique=True)

The above, coupled with the following manual migration on the number field:

from django.db import migrations


class Migration(migrations.Migration):

    dependencies = [
        ("billing", "0003_my_migration"),
    ]

    operations = [
        migrations.RunSQL(
            "ALTER TABLE invoice ALTER COLUMN number ADD GENERATED BY DEFAULT AS IDENTITY;",
            reverse_sql="ALTER TABLE invoice ALTER COLUMN number DROP IDENTITY IF EXISTS;"
        ),
    ]