Firebase realtime database, put data only if the user not exist-CodePudding

I have to check that if the user who clicked already participates in this event then he is inside the "Partecipanti" field that Toast that I put comes out otherwise it is inserted in the "Partecipanti". How can i do? i put below the code i have made

if(dataSnapshot.exists()) return value null. Not show the Toast, Why?

Database example:https://imgur.com/a/QgeQspW

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


    String uid = FirebaseAuth.getInstance().getCurrentUser().getUid();

    LatLng latLon = marker.getPosition();

    //Cycle through places array
    for(Incontro incontro : Incontri) {
        if (latLon.equals(incontro.getLatlng())) {

            DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Incontro")
                    .child(incontro.getIncontroidId())
                    .child("Partecipanti");

            Query query = reference.child("").equalTo(uid);
            query.addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                    if(dataSnapshot.exists()) {
                        Toast.makeText(Cercaincontro.this, "You are already attending this event", Toast.LENGTH_SHORT).show();

                    }else{

                        DatabaseReference reference1 = FirebaseDatabase.getInstance().getReference("Incontro").child(incontro.getIncontroidId());
                        HashMap<String, Object> hashMap1 = new HashMap<>();
                        hashMap1.put("userid", firebaseUser.getUid());
                        reference1.child("Partecipanti").child(uid).setValue(true);
                        reference1.child("partecipanti").setValue(ServerValue.increment(1));
                    }

                }
                @Override
                public void onCancelled(DatabaseError databaseError) { throw databaseError.toException();

                }
            });

        }
    }

}

CodePudding user response：

When you're using:

Query query = reference.child("").equalTo(uid);

You're checking an empty string against a UID, which obviously returns no results. If you want to check if a particular UID exists under the Partecipanti node, there is no need to use a query, simply build a reference and check if the child exists:

DatabaseReference uidRef = reference.child(uid);
ValueEventListener valueEventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        if(dataSnapshot.exists()) {
            Log.d("TAG", "The UID already exists.");
        } else {
            Log.d("TAG", "The UID doesn't exist.");
        }
    }

    @Override
    public void onCancelled(@NonNull DatabaseError error) {
        Log.d("TAG", error.getMessage()); //Never ignore potential errors!
    }
};
uidRef.addListenerForSingleValueEvent(valueEventListener);