@classmethod
def get_mains_powered_outlets(cls, session):
    """Wall-powered outlets

        Args:
            session: database session
        Returns:
            list: of outlet keys powered by the mains
        """
    results = session.run(
        """
            MATCH (outlet:Outlet) WHERE NOT (outlet)-[:POWERED_BY]->(:Asset) RETURN outlet.key as key
            """
        )
    return list(map(lambda x: x.get('key'), results))
