join.py

def join_to(self, table1, table2, field1, field2, queryset, alias='', left=True):
    def extra_join_cond(where_class, alias, related_alias):
        if (alias, related_alias) == ('[sys].[columns]',
                                      '[sys].[database_permissions]'):
            where = '[sys].[columns].[column_id] = ' \
                    '[sys].[database_permissions].[minor_id]'
            children = [ExtraWhere([where], ())]
            wh = where_class(children)
            return wh
        return None

    dpj = ForeignObject(
        to=table2,
        on_delete=lambda: None,
        from_fields=[None],
        to_fields=[None],
        rel=None,
        related_name=None
    )
    dpj.opts = Options(table1._meta)
    dpj.opts.model = table1
    dpj.get_joining_columns = lambda: ((field1, field2),)
    dpj.get_extra_restriction = extra_join_cond

    is_left_join = True if left else False
    dj = Join(table2._meta.db_table, table1._meta.db_table, 'T', "JOIN", dpj, is_left_join)

    ac = queryset._clone()
    ac.query.join(dj)
    alias and setattr(dj, 'table_alias', alias)
    return ac

# example of usecase   
q = Principals.objects.using(db_name).filter(type__in=["'S'", "'U'", "'G'", "'R'", "'A'"])
.annotate(
  UserName=Case(When(server_name=None,
                    then=CastFunc(account_name_, output_field=CharField())),
                default=CastFunc(server_name_, output_field=CharField()),
                output_field=CharField()),
  Grantor=Col('T10', DatabasePrincipals._meta.get_field('name'),
              output_field=CharField()))
  ... other fields
  )
  ac = self.join_to(DatabasePrincipals, ServerPrincipals, 'sid', 'sid', q)
  ac = self.join_to(DatabasePermissions, DatabasePrincipals, 'grantor_principal_id', 'principal_id', ac, 'T10')