Save multiple data with same uid to Firebase database

Question

I'm working in Android Studio on an app that uses Firebase for signing in and store data to its realtime database. I'm using the default firebase database rules, so only authenticated users can write/read in the database.

I've an activity (AddMarker) that saves two editText values and one LatLong value (this one from another activity) to the database. The code is actually working and i can see data stored in it in this way: DB structure

The problem is that everytime i save (in AddMarker Activity), the previous data linked to that id, gets replaced by the new one.
Instead i would like to store in the DB multiple values with the same id like this for example: DB structure 2

This is my code

public class AddMarker extends AppCompatActivity implements View.OnClickListener {

    //Save flag
    public static boolean isSaved;

    //Database
    private DatabaseReference databaseReference;
    FirebaseAuth firebaseAuth;
    private FirebaseAuth mAuth;

    //UI
    private EditText tipo, marca;
    private Button saveMarker;

    public LatLng ll;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_add_marker);

        //DB
        mAuth = FirebaseAuth.getInstance();
        databaseReference = FirebaseDatabase.getInstance().getReference();


        //Initialize views
        tipo = (EditText) findViewById(R.id.type);
        marca = (EditText) findViewById(R.id.specInfo);
        saveMarker = (Button) findViewById(R.id.save_btn);

        //Initialize isSaved
        isSaved = false;

        //Set button listener
        saveMarker.setOnClickListener(this);

        //get Position
        ll = getIntent().getParcelableExtra("POSITION");

    }

    private MarkerOptions saveMarkerInfo(){

        String tipo_str = tipo.getText().toString().trim();
        String marca_str = marca.getText().toString().trim();
        LatLng pos = ll;

        MarkerInfo markerInfo = new MarkerInfo(tipo_str,marca_str, pos);

        FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();


        MarkerOptions marker = new MarkerOptions()
                .position(ll)
                .anchor(0.0f, 1.0f)
                .title("prova")

        //Add data to DB
        databaseReference.child(user.getUid()).setValue(markerInfo);


        Toast.makeText(this,"infos saved",Toast.LENGTH_LONG).show();

        finish();

        return marker;
    }

    @Override
    public void onClick(View v) {

        if(v == saveMarker){
            isSaved = true;
            MarkerOptions mo=saveMarkerInfo();

            MainActivity.mMap.addMarker(mo);

        }

    }
}

Answer 1

you cannot create two nodes with same Id instead you can create a nested structure inside each userId. and setValues accordingly like this.

 databaseReference.child(user.getUid()).push().setValue(markerInfo);

this will create childs under singleUserId with your multiple data

for more you can refer this