Checking if a user already exists at firebase realtime database

Question

I get the Alex Mamo's Solution in order to check whether a unique value exists in my database but the snapshot gets always null. The solution is here:

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference userNameRef = rootRef.child("Users").child("Nick123");
ValueEventListener eventListener = new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
    if(!dataSnapshot.exists()) {
        //create new user
    }
}

    @Override
    public void onCancelled(DatabaseError databaseError) {}
};
userNameRef.addListenerForSingleValueEvent(eventListener);

And my database looks like this:

users
    (auto-generated push token ID)
        fullname
        username
        gender
        ...

I don't know whether it's the right method but I used push() method for adding the user object to the database and this method generates push token ID like above.

So, in order not to create duplicate users, what should I change with the above solution? Is there a better method to do this? Maybe like checking the firebase uid with some firebase auth methods?

UPDATE My database screenshot: As you see the string just below 'users' is not the same as user's uid.

Answer 1

If you have this database:

users
(auto-generated push token ID)
    fullname
    username
    gender

Then to check if user exists you need to do the following:

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference userNameRef = rootRef.child("users");
Query queries=userNameRef.orderByChild("fullname").equalTo("Nick123");
ValueEventListener eventListener = new ValueEventListener() {
  @Override
public void onDataChange(DataSnapshot dataSnapshot) {
   if(!dataSnapshot.exists()) {
       //create new user
    }
 }

    @Override
   public void onCancelled(DatabaseError databaseError) {}
};
queries.addListenerForSingleValueEvent(eventListener);

First if you have users lowercase letter, then inside child add users, then you need to use a query to be able to check if the fullname is equal to Nick123.

---

Usually the database is structured like this:

users
  userId
     fullname: Nick123

The attributes are key:value, the key is the identifier and Nick123 is the value, so inside the child() method you need to write the identifier which is fullname and not Nick123.

Then since you are able to get the userId, you can do this:

FirebaseUser currentFirebaseUser =FirebaseAuth.getInstance().getCurrentUser();
String userId=currentFirebaseUser.getUid();
 DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference userNameRef = rootRef.child("users").child(uid).child("fullname");
userNameRef.addValueEventListener(new ValueEventListener(){...}

Answer 2

This is not the best practice when it comes to save user details into the database. First, you should implement Firebase Authentication and then in order to check if a user exist in your database, you should use the following lines of code:

String uid = FirebaseAuth.getInstance().getCurrentUser().getUid();
DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference uidRef = rootRef.child("users").child(uid);
ValueEventListener eventListener = new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
    if(!dataSnapshot.exists()) {
        //create new user
    }
}

    @Override
    public void onCancelled(DatabaseError error) {
        Log.d("TAG",error.getMessage()); //Don't ignore potential errors!
    }
};
uidRef.addListenerForSingleValueEvent(eventListener);

So the key for solving this problem is to call the child() method and pass the UID as an argument instead of calling the push() method that generates an random unique identifier.