How to work around lack of support for foreign keys across databases in Django

Question

I know Django does not support foreign keys across multiple databases (originally Django 1.3 docs)

But I\'m looking for a workaround.

What doesn\'t work

Answer 1

This solution is originally written for one managed database with migrations and one or more legacy databases with models Meta managed=False connected at database level to the same database. If a db_table option contains a database name plus table name quoted correctly by ' ` ' (MySQL) or by ' " ' (other db), e.g. db_table = '"DB2"."table_b"', then it is not quoted any more by Django. Queries are compiled by Django ORM correctly, even with JOINs:

class TableB(models.Model):
    ....
    class Meta:    
        db_table = '`DB2`.`table_b`'    # for MySQL
        # db_table = '"DB2"."table_b"'  # for all other backends
        managed = False

Query set:

>>> qs = TableB.objects.all()
>>> str(qs.query)
'SELECT "DB2"."table_b"."id" FROM DB2"."table_b"'

That is supported by all db backends in Django.

(It seems that I started a bounty on a duplicate new question where my answer continues.)