def relation(self, relation_name):
        return self._relations.get(relation_name, None)
Called when we receive a MSG_DEBUG message.  Payload::
dataset/ETHPy150Open coursera/dataduct/dataduct/database/database.py/Database.relation--crossover with--dataset/ETHPy150Open twisted/twisted/twisted/conch/ssh/transport.py/SSHTransportBase.ssh_DEBUG