Firebase - Forcing unique usernames

Question

Following information on similar questions on Stack I've ended up with the following:

Database:

{
  "user" : {
    "xbStJyCtfzf472" : {
      "avatar" : "avatar",
      "email" : "[email protected]",
      "fbId" : 10154200898,
      "lastLogin" : "Tue Mar 14 22:31:36 GMT+01:00 2017",
      "registerDate" : "Tue Mar 14 22:31:36 GMT+01:00 2017",
      "uid" : "xbStJyCwY5Ttfzf472",
      "username" : "myName"
    }
  },
  "usernames" : {
    "myName" : "xbStJyCtfzf472"
  }
}

Rules:

{
  "rules": {
    ".read": "auth != null",
    ".write": "auth != null",

    "users": {
    "$uid": {
        ".write": "auth !== null && auth.uid === $uid",
        ".read": "auth !== null && auth.provider === 'password'",
        "username": {
        ".validate": "
            !root.child('usernames').child(newData.val()).exists() ||
            root.child('usernames').child(newData.val()).val() == $uid"
            }
        }
        }
  }
}

Android Java Code:

public void addUserToDatabase(final long fbId){
        final FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
        if (user != null) {
            final String username = "myName";

            //add the user
            mDatabase.child("usernames").child(username).runTransaction(new Transaction.Handler() {
                @Override
                public Transaction.Result doTransaction(MutableData mutableData) {
                    if (mutableData.getValue() == null) {
                        mutableData.setValue(user.getUid());
                        return Transaction.success(mutableData);
                    }

                    return Transaction.abort();
                }

                @Override
                public void onComplete(DatabaseError firebaseError, boolean commited, DataSnapshot dataSnapshot) {
                    if (commited) {
                        String email = user.getEmail();
                        String uid = user.getUid();
                        String avatar = "avatar1";
                        long mFbId = fbId;
                        String registered = Calendar.getInstance().getTime().toString();
                        String lastLogin = Calendar.getInstance().getTime().toString();

                        Log.w("AUTH", "username saved");
                        mDatabase.child("user").child(uid).setValue(new User(username, email, uid, avatar, mFbId, lastLogin, registered));
                    } else {
                        Log.w("AUTH", "username exists");
                    }
                }
            });
        }
    }

This works, since if I try to register accounts with the same userName, nothing is created in my database. However, A user is created in my Authentication/Users. I can see this leading to all kinds of issues.

How should I be forcing unique usernames in my database in a way that does not register the user until the check has been made?

EDIT: To clarify, I can detect that the username is taken, and prompt the user to input another. However, what if the user gets a call/exits my app while doing this, then their email will be authenticated (i.e they cant register again), but they won't have a User-object in my database, so they will never be able to use my app.

Answer 1

Short Answer:

First register user, then prompt them to put in their username, when they do, create their User-object in your database.

Solution to problem:

If a user for some reasons exits the app while deciding on a username, leading their email to be stored in the authentication-table without a User-object in the database, we have a problem. To solve this, whenever a user signs in, we can check if their UID exists in the User-table, if it does, great! If not, then we prompt them to enter a username again, and then create the User-object.