Retrieve users in a two-way relationship with Firebase - Android

Question

I'm trying to figure out a way of retrieving all users that follow me. Let's say I'm user1, and user2 and user3 are my followers. How can I only retrieve them instead of retrieving all users and filter them on the client side?

This is how my structure looks like:

{
  "followers" : {
    "user1" : {
      "user2" : true,
      "user3" : true
    }
  },

  "following" : {
    "user2" : {
      "user1" : true
    },
    "user3" : {
      "user1" : true
    }
  },

  "users" : {
    "user1" : {
      "firstName" : "User One"
    },
    "user2" : {
      "firstName" : "User Two"
    },
    "user3" : {
      "firstName" : "User Three"
    }
  }
}

I don't have any code yet because I don't know how to even start. Any help is appreciated. Thanks for reading.

EDIT: This is how I got it working. I don't know if it's the best option, but it works for now.

ChildEventListener childEventListener = new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String previousChildName) {
                Log.d(TAG, "onChildAdded:" + dataSnapshot.getKey());
                // A new comment has been added, add it to the displayed list
                String uid = dataSnapshot.getKey();

                mDatabase.child("users").child(uid).addListenerForSingleValueEvent(
                        new ValueEventListener() {
                            @Override
                            public void onDataChange(DataSnapshot dataSnapshot) {
                                // Get user value
                                User user = dataSnapshot.getValue(User.class);
                                users.add(user);
                                notifyDataSetChanged();
                            }

                            @Override
                            public void onCancelled(DatabaseError databaseError) {
                                Log.w(TAG, "getUser:onCancelled", databaseError.toException());
                            }
                        });
            }

            @Override
            public void onChildChanged(DataSnapshot dataSnapshot, String previousChildName) {
                Log.d(TAG, "onChildChanged:" + dataSnapshot.getKey());
                // A comment has changed, use the key to determine if we are displaying this
                // comment and if so displayed the changed comment.

                //Comment newComment = dataSnapshot.getValue(Comment.class);
                String commentKey = dataSnapshot.getKey();
                System.out.println("Changed: "+commentKey);
                notifyDataSetChanged();
            }

            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {
                Log.d(TAG, "onChildRemoved:" + dataSnapshot.getKey());
                // A comment has changed, use the key to determine if we are displaying this
                // comment and if so remove it.

                String commentKey = dataSnapshot.getKey();
                System.out.println("Removed: "+commentKey);
                notifyDataSetChanged();
            }

            @Override
            public void onChildMoved(DataSnapshot dataSnapshot, String previousChildName) {
                Log.d(TAG, "onChildMoved:" + dataSnapshot.getKey());
                // A comment has changed position, use the key to determine if we are
                // displaying this comment and if so move it.

                //User user = dataSnapshot.getValue(User.class);
                String commentKey = dataSnapshot.getKey();
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                Log.w(TAG, "postComments:onCancelled", databaseError.toException());
                Toast.makeText(context, "Failed to retrieve followers.", Toast.LENGTH_SHORT).show();
            }
        };
        mDatabase.child("followers").child(currentUser.getUid()).addChildEventListener(childEventListener);

Use addListenerForSingleValueEvent() method for retrive data.Refer this link https://firebase.google.com/docs/database/android/retrieve-data — Narender Reddy, Aug 27 '16 at 03:35
@Narender Hey, thanks for the fast reply, but this doesn't actually solve my problem. The problem I'm having is that there isn't a way of filtering data to get my followers using a single query. — Guz F, Aug 27 '16 at 03:53
no you have to repeat the loop means you will get chield items then repeat the cheild of every item to get sub chield items — Narender Reddy, Aug 27 '16 at 04:18
@Narender I've edited my code. Is that what you meant? (Btw, this is working) — Guz F, Aug 27 '16 at 22:35

score 1 · Answer 1 · edited May 23 '17 at 12:08

1

You can get the follower's user IDs in one read (not even a query) from ref.child("followers").child(user.getUid()).

Then you'll indeed need to loop to retrieve each follower from under /users/$followerId. This is not as expensive as you probably think, since the Firebase client efficiently pipelines the requests to the server. See Speed up fetching posts for my social network app by using query instead of observing a single event repeatedly.

edited May 23 '17 at 12:08

Community

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1

answered Aug 27 '16 at 04:35

Frank van Puffelen

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Hey, thanks for your reply. I've edited my original post and that is how my code looks like. It works for now, but I'm not quite sure if that's the ideal option for my request. – Guz F Aug 27 '16 at 22:32

Retrieve users in a two-way relationship with Firebase - Android

1 Answers1