@classmethod
def get_controller_count(cls, session, server_key):
    """Get number of controllers per server
        Args:
            session:  database session
            server_key(int): key of the server controller belongs to
        Returns:
            int: controller count
        """
    results = session.run(
        """
            MATCH (:Asset { key: $key })-[:HAS_CONTROLLER]->(ctrl:Controller) RETURN count(ctrl) as ctrl_count
            """
        , key=server_key)
    record = results.single()
    return int(record.get('ctrl_count')) if record else None
