[FIXED] Firebase realtime Database Hashmap overwrites data

Issue

In short, when I click on the marker it must register the user id of the user who clicked on "Partecipanti". The problem is that if the first user is saved regularly when a second user clicks, this overwrites the first one. How can I solve it?

 @Override
public void onInfoWindowClick(@NonNull Marker marker) {

    LatLng latLon = marker.getPosition();

    for(Incontro incontro : Incontri) {
        if (latLon.equals(incontro.getLatlng())) {

            if(){
                Toast.makeText(Cercaincontro.this, "You are already attending this event", Toast.LENGTH_SHORT).show();
            }
            else {

                DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Incontro").child(incontro.getIncontroidId());
                HashMap<String, Object> hashMap1 = new HashMap<>();
                String uid = FirebaseAuth.getInstance().getCurrentUser().getUid();
                hashMap1.put("userid", firebaseUser.getUid());

                reference.child("Partecipanti").child(uid).setValue(true);

            }

        }
    }

}

Solution

While @AliasCartellano's solution to pushing data into the Realtime Database will work, please note a pushed ID it's not the best option to identify a user. Why? Because every time you call push(), another unique key is generated.

Since I see in your screenshot that you store some long IDs, other than the pushed IDs, the ones that start with - (minus), I assume you're using authentication. That being said, a more recommended way to store user data would be like this:

Firebase-root
  |
  --- users
       |
       --- $uid
            |
            --- //user data

And to add data to such a structure, you should use the following lines of code:

String uid = FirebaseAuth.getInstance().getCurrentUser().getUid();
reference.child("Partecipanti").child(uid).setValue(hashMap1);

This means, that each user will write data under a separate location since each UID is unique. So in this way, you cannot overwrite the data.

Edit:

Firebase-root
  |
  --- users
       |
       --- $uid: true
       |
       --- $uid: true

And in code:

reference.child("Partecipanti").child(uid).setValue(true);

Edit2:

To be able to write an object that looks like this, please use the following lines code:

Map<String, Object> data = new HashMap<>();
data.put("addressorganizz", "messina");
data.put("cittaorg", "messin");
//As as much data as you need.
reference.child("Partecipanti").child(uid).setValue(data);

Edit3:

//Data
Map<String, Object> data = new HashMap<>();
data.put("addressorganizz", "messina");
data.put("cittaorg", "messin");

String uid = FirebaseAuth.getInstance().getCurrentUser().getUid(); 
DatabaseReference reference = FirebaseDatabase.getInstance()
    .getReference("Incontro")
    .child(incontro.getIncontroidId())
    .child(uid);  
reference.setValue(data);

Answered By - Alex Mamo
Answer Checked By - Candace Johnson (JavaFixing Volunteer)