Check if user Exists in Firebase Database

Question

I'm trying to check it in Database with an addListenerForSingleValueEvent method, but when I debug the code the listener is skipped, this is my code of the listener.

    mTheReference = FirebaseDatabase.getInstance().getReference();
    mUsersReference = mTheReference.child("Users");

   mUsersReference.addListenerForSingleValueEvent(new ValueEventListener() {
                        @Override
                        public void onDataChange(DataSnapshot dataSnapshot) {

                                for (DataSnapshot postSnapshot : dataSnapshot.getChildren()) {
                                    User post = postSnapshot.getValue(User.class);
                                    String username = post.getUsername();

                                    if (username.equals(mUserView.getText().toString())) {
                                        alreadyRegisteredAccount++;
                                    }
                                }

                        }

                        @Override
                        public void onCancelled(DatabaseError databaseError) {

                        }
                    });
if (alreadyRegisteredAccount != 0) {
                        mUserView.setError("Usuario ya registrado, intenta de nuevo.");
                    } else {
                        User user = new User();
                        user.setUsername(mUserView.getText().toString());
                        user.setPassword(mPasswordView.getText().toString());
                        mUsersReference.push().setValue(user).addOnCompleteListener(new OnCompleteListener<Void>() {
                            @Override
                            public void onComplete(@NonNull Task<Void> task) {
                                Toast.makeText(getApplicationContext(), "YAAAY!", Toast.LENGTH_LONG).show();
                            }
                        });
                    }

I don't know why its skipped, I need something that let me check if a user exists once the user tries to register a new username.This is the data tree:

{
 users:
  {
    -KjGdQAZEfJ-qR5pM0Ko:
     {
       username :  fmigg
       password :  broxton1
     }
    -KjGq7AghI9ftiX6Ih-q:
     {
       username :  aston
       password :  creative123
     }
  }
}

Answer 1

Your DatabaseReference is wrong. Please change this line:

mUsersReference = mTheReference.child("Users");

with

mUsersReference = mTheReference.child("Users").child(userId);

In which userId is the unique id generated by the push() method.

Hope it helps.