firebase check if child exists

Question

So I have my backend using firebase. What I'm aiming to do is to add user matches to a user id. However, when the user initially signs up, they have no matches. What I'm trying to do is to check whether or not the "match" child exists within a user child, if not a list child is created and the first match is stored. However, if it already exists, the match is simply added. Here's my code:

    public void setMatch(final String match){
    final Firebase ref = new Firebase("FIREBASEURL");
    final Firebase userRef = ref.child("Flights").child(userName);
    userRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            System.out.println("does the child exist? " + dataSnapshot.child("matches").exists());
            if(!dataSnapshot.child("matches").exists()){
                ArrayList<String> matches = new ArrayList<String>();
                matches.add(match);
                Firebase matchesRef = userRef.child("matches");
                matchesRef.setValue(matches);
                userRef.removeEventListener(this);
            }else if(dataSnapshot.child("matches").exists()){
                Map<String, Object> matches = new HashMap<>();
                matches.put("matches", match);
                userRef.child("matches").push().setValue(matches);
                userRef.removeEventListener(this);
            }
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {

        }
    });
}

Currently, the value is being added twice (the else if is called twice if the field already exists/its called if it doesn't). I'm not sure what I'm doing wrong.

Answer 1

This sounds pretty overcomplicated. In the Firebase Database, it's often best to separate read and write operations as much as possible. And while push ids are a great way to store data in a chronological way; if items have a natural key, it is often better to store them under that key.

For example if your String match is really a String matchId, you can ensure that each match is present at most once by using matchId as the key.

userRef.child("matches").child(matchId).setValue(true);

This operation is idempotent: it will give the same result no matter how often you run it.

You'll note that I don't check of matches already exists: the Firebase Database automatically creates all nodes that are needed to store the value and it automatically removes all nodes that have no values under them.

Answer 2

It looks like you create the field if it doesn't exist in the if block, and then test to see if that field (which was just created) exists, and it now does, so it adds it again. The removeEventListener call will remove the listener, but will not stop the current code from completing.

Try:

if(!dataSnapshot.child("matches").exists()){
            ArrayList<String> matches = new ArrayList<String>();
            matches.add(match);
            Firebase matchesRef = userRef.child("matches");
            matchesRef.setValue(matches);
            userRef.removeEventListener(this);
            return; 
        }else if(dataSnapshot.child("matches").exists()){
            Map<String, Object> matches = new HashMap<>();
            matches.put("matches", match);
            userRef.child("matches").push().setValue(matches);
            userRef.removeEventListener(this);
        }

Adding the return statement should quite the current call, and still disable the Listener as you intended.