How to do order by child inside another child Firebase

问题

I am trying to get all data ordered asc and desc by child value that inside another child. In main scope of class:

FirebaseDatabase database = FirebaseDatabase.getInstance();
DatabaseReference dbRef = database.getReference("university");

i have data structure like this

{
"university": {
    "13": {
      "name": "value"
      "exam": {
        "exam_date": "value"
        "reg_date": "value"
      }
    }
    "15": {
      "name": "value"
      "exam": {
        "exam_date": "value"
        "reg_date": "value"
      }
    }
    "20": {
      "name": "value"
      "exam": {
        "exam_date": "value"
        "reg_date": "value"
      }
    }
}

}

First I try this way Query q=dbRef.orderByChild("13/exam/exam_date") but it is getting data like when i use ValueEventListener with dbRef directly without ordering.

回答1:

To solve this, please change the following line of code:

Query q=dbRef.orderByChild("13/exam/exam_date")

with

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
Query query = rootRef.child("university").orderByChild("exam/exam_date");

回答2:

 firebaseDatabaseRef.child(AppConstants.RootNode.userRoomGroupIds).child(myUserId).orderByChild(AppConstants.FireBaseRoom.LAST_MESSAGE+"/"+AppConstants.LASTMESSAGE.MESSAGE_TIME_STAMP).startAt(System.currentTimeMillis());

This query give data from next to my current milisec so i use orderByChild in which give refrence to particular child /lastmessage/timeStamp

来源：https://stackoverflow.com/questions/49445352/how-to-do-order-by-child-inside-another-child-firebase