Inserting The UID into Firebase Database Only once

Question

In the main activity of Android Studio, immediately after anonymously signing in into firebase, I have this line of code:

        FirebaseDatabase.getInstance().getReference().child("Online Users").push().setValue(currentUser.getUid().toString());

So, the UID of current user will be saved into firebase under the child "Online Users". Good. Problem is, when I open the app for the second or third time, or if I navigate from another activity to the main activity, this line of code gets executed again, and now I have several identical entries in the firebase. I mean, one UID has been pushed into firebase several times. But I don't want that. I want only one copy of each UID there. How can I achieve that?

Answer 1

Whenever you want a specific value to be unique in the Firebase Database, model it as the key of a list.

In your case that means that instead of calling push() you actually use the UID as the key:

FirebaseDatabase.getInstance().getReference().child("Online Users").child(currentUser.getUid().toString()).setValue(true);

Now whenever the user comes online again, they're just writing true to the same location again.

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Note that the same user can come online from multiple devices. If you are trying to build a presence system, tracking what users are online, I recommend studying the sample presence system in the Firebase documentation which handles the edge cases correctly.