Django Custom Model ForeignKey Field for Spanning Databases

related.py

from django.core import exceptions
from django.db.models.fields.related import ForeignKey
from django.db.utils import ConnectionHandler, ConnectionRouter

connections = ConnectionHandler()
router = ConnectionRouter()


class SpanningForeignKey(ForeignKey):

    def validate(self, value, model_instance):
        if self.rel.parent_link:
            return
        # Call the grandparent rather than the parent to skip validation
        super(ForeignKey, self).validate(value, model_instance)
        if value is None:
            return

        using = router.db_for_read(self.rel.to, instance=model_instance)
        qs = self.rel.to._default_manager.using(using).filter(
            **{self.rel.field_name: value}
        )
        qs = qs.complex_filter(self.get_limit_choices_to())
        if not qs.exists():
            raise exceptions.ValidationError(
                self.error_messages['invalid'],
                code='invalid',
                params={
                    'model': self.rel.to._meta.verbose_name, 'pk': value,
                    'field': self.rel.field_name, 'value': value,
                },  # 'pk' is included for backwards compatibility
            )

I'm trying to use this class to allow for a lookup of a user in auth_user in a 'default' DB , different from my application DB. ('test2' here).
the table I created looks like:

CREATE TABLE IF NOT EXISTS "test2_m1" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "description" text NOT NULL, "user_id" integer NOT NULL ); -- no foreign key

But when I run these statements, Django generates the following query wth an INNER JOIN to auth_user, and SpanningForeignKey.validate() is not called at all (I added a debug line to test):

from test2.models import *
M1.objects.get(user__username = 'xyz')
db_for_read: test2 ---> test2
ZZZZ SELECT "test2_m1"."id", "test2_m1"."user_id", "test2_m1"."description" FROM "test2_m1" INNER JOIN "auth_user" ON ("test2_m1"."user_id" = "auth_user"."id") WHERE "auth_user"."username" = xyz

The problem here is that the underlying Django code still wants to form a query with an INNER JOIN on the app-specific DB, not the default DB, expecting "auth_user" to be in that DB.

My questions:

1. How do I get Django to NOT do that INNER JOIN on "auth_user" , especially when I specified no foreign key in the app's DB ? Shouldn't the SpanningForeignKey class prevent this from happening ?
2. If SpanningForeignKey.validate() won't do that for me, what other method(s) must I override ?