如何从firebase检索特定的数据列表？

Question

Is there any possible way to get result from child of child. I'm looking for multiple answers on a question.!

"questions" : {
    "Q1(Random ID )" : {
      "description" : "Deail of Question",
      "idQuestion" : "Q1",
      "time" : "17 Mar 2017 16:18:12",
      "title" : "Title Of Question",
      "user_id" : "bXCeW6jfidbHuMCCCMkDGWcGZRS2"
    },
{
  "Answer" : {
    "81d19e85-8c6c-4824-9fb1-61a7cd316e32" : {
      "dateTime" : "17 Mar 2017 16:22:17",
      "Answer" : "Answer",
      "ProfessorId" : "da587c89-f001-43f2-984d-e6dbac491acc",
      "questionId" : "Q1"
    },
    "e2ec0556-4420-4d2f-b346-0b5cebeb1ebe" : {
      "dateTime" : "17 Mar 2017 16:55:27",
      "Answer" : "Answer 2 on Question 1",
      "answerId" : "0d20d6df-907d-4133-be53-ca6acf6c2ad1",
      "questionId" : "Q1"
    }
  },

nodes that have been generated randomly. ! want to do something like this .!

mdatabaseReference.child("Answer").child(* Here Im Using "*" Like we used to do in SQl to get all data I knw its not a SQL but its just for conecpt).equalTo(QID).addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

                if (Answer_List.size() > 0)
                    Answer_List.clear();


                for (DataSnapshot postsnapshot : dataSnapshot.getChildren()) {

Instead of using using postSnapshot.getKey().equals("QID")

    if(postSnapshot.getKey().equals("QID"))  
 Answertext = postSnapshot.getValue().toString();

It will make performance slower I'm looking for some thing faster.!

looking for something like this

 mdatabaseReference.child("Answer").child("*").getKey().equals(QID).addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

Answer 1

You're looking for Firebase Database queries .

mdatabaseReference
  .child("Answer")
  .orderByChild("questionId")
  .equalTo("Q1")
  .addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
      for (DataSnapshot answerSnapshot: dataSnapshot.getChildren()) {
        System.out.println(answerSnapshot.child("Answer").getValue());
      }
    }
    ...

However, while this will work, I highly recommend that you change your data model to make this type of query more scaleable. Since answers fall under questions, you should model that hierarchy. While you should not mix questions and answers, you should consider modeling the answers under the question ID:

"questions" : {
    "Q1(Random ID )" : {
      "description" : "Deail of Question",
      "idQuestion" : "Q1",
      "time" : "17 Mar 2017 16:18:12",
      "title" : "Title Of Question",
      "user_id" : "bXCeW6jfidbHuMCCCMkDGWcGZRS2"
    },
}
"Answer" : {
  "Q1": {
    "81d19e85-8c6c-4824-9fb1-61a7cd316e32" : {
      "dateTime" : "17 Mar 2017 16:22:17",
      "Answer" : "Answer",
      "ProfessorId" : "da587c89-f001-43f2-984d-e6dbac491acc",
    },
    "e2ec0556-4420-4d2f-b346-0b5cebeb1ebe" : {
      "dateTime" : "17 Mar 2017 16:55:27",
      "Answer" : "Answer 2 on Question 1",
      "answerId" : "0d20d6df-907d-4133-be53-ca6acf6c2ad1",
    }
  }
}

With this structure you can get the answers for Q1 with a direct lookup instead of a query (on an every growing list):

mdatabaseReference
  .child("Answer")
  .child("Q1")
  .addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
      for (DataSnapshot answerSnapshot: dataSnapshot.getChildren()) {
        System.out.println(answerSnapshot.child("Answer").getValue());
      }
    }
    ...