我的用户节点中有一个名为subscribedTo的数组。现在我想在用户订阅时向该数组附加一些pushID's。但是pushID被替换而不是附加。如何将推送ID附加到数组？架构"tester@gmail,com":{"email":"tester@gmail,com","hasLoggedInWithPassword":true,"name":"tester","subscribedTo":["-KFPi5GjCcGrF-oaHnjr"],"timestampJoined":{"timestamp":1459583857967}}代码publicvoidonSubscribe(Viewv