Firebase获取最后一个id - limitToLast无效 - android

Question

我正在尝试获取我的消息节点的最后一个ID,但它没有得到最后一个.例如,我的最后一个id是12,但它总是返回项目10的id.我的代码是这样的:

final Query query = firebase.orderByChild("messageId").limitToLast(1);
query.addListenerForSingleValueEvent(new ValueEventListener() {

   public void onDataChange(DataSnapshot dataSnapshot) { 

      Log.i("MessagesFrag","snapshot: "+dataSnapshot.getValue()+""); 

   }

   public void onCancelled(FirebaseError firebaseError) {}

});

它返回如下内容:

{10={chatId=6, time=Wed Apr 13 22:20:43 EDT 2016, message=hi, user=123, messageId=10}}

它应该返回:

{12={chatId=6, time=Wed Apr 13 22:29:02 EDT 2016, message=I am fine, user=1234, messageId=12}}

我的数据是这样的:

messages
  9:
    chatId: "6"
    message: "hi"
    messageId: "9"
    time: "Wed Apr 13 22:29:02 EDT 2016"
    user: "123"
  10:
    chatId: "6"
    message: "hello"
    messageId: "10"
    time: "Wed Apr 13 22:29:02 EDT 2016"
    user: "1234"
  11:
    chatId: "6"
    message: "how are you?"
    messageId: "11"
    time: "Wed Apr 13 22:29:02 EDT 2016"
    user: "123"
  12:
    chatId: "6"
    message: "I am fine"
    messageId: "12"
    time: "Wed Apr 13 22:29:02 EDT 2016"
    user: "1234"

除此之外,当我将"LimitToLast"更改为"LimitToFirst"(它应该得到第一个)时,它不会获得我拥有的第一个元素(节点1),它也获得节点10.

有谁知道可能是什么问题？

Answer 1

我认为问题是orderByChild，由于消息是以数字方式排序的，因此不需要再次排序。尝试这个 ：

 FirebaseDatabase database = FirebaseDatabase.getInstance();
        // change this to your databae ref
        final DatabaseReference messages = database
                .getReference("XXXX")
                .child("XXXX"); // change this to your databae ref

        messages .limitToLast(1).addListenerForSingleValueEvent(new ValueEventListener() {

            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

                if (dataSnapshot.exists()) {
                    // access last message
                    DataSnapshot messageSnapShot= dataSnapshot.getChildren().iterator().next();
                }
            }
            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });