'Returns a pair condition used for joining and subquery pushdown. The
condition is something that responds to as_sql(qn, connection) method.
Note that currently referring both the \'alias\' and \'related_alias\'
will not work in some conditions, like subquery pushdown.
A parallel method is get_extra_descriptor_filter() which is used in
instance.fieldname related object fetching.'

def get_extra_restriction(self, where_class, alias, related_alias):

    return None
