如何使用Java从Firebase检索新添加的子代的父键

Question

I want to retrieve "7753400075_Canada_twinpalms_||_twinpalms_20170824001545_F5BA7F33AD51CE283332001BEC409A46" key, How to do so? Here is my code...

ref.addValueEventListener(new ValueEventListener() { //addListenerForSingleValueEvent
                @Override
                public void onCancelled(DatabaseError arg0) {
                }
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    Object response = dataSnapshot.getValue();
                    System.out.println("read data :"+response);
                }
            });
            ref.child("chatmessage").child("devicetoken").addChildEventListener(new ChildEventListener() {

                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String previousChildName) {
//                  Log.d(TAG, "onChildAdded:" + dataSnapshot.getKey());
//                  System.out.println("previousChildName :"+previousChildName);

                    }
                }
});

Here is the Firebase database example:

I tried using child data snapshot method but did not worked. It returns all data. But i need to get only the new child added parent. Any help for this will be much appreciated.

Answer 1

To solve this, you need to filter your results. Assuming that chatmessage is a direct child of your Firebase database root, please use the following code:

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference deviceTokenRef = rootRef.child("chatmessage").child("devicetoken");
Query query = deviceTokenRef.orderByKey().limitToLast(1);
ValueEventListener eventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            String key = ds.getKey();
            Log.d("TAG", key);
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {}
};
query.addListenerForSingleValueEvent(eventListener);