Query for document in subcollection firestore

Question

I have the following structure of my database:

Inside the Location collection there is a document and in it there is Lat, Lon coordinates:

My final goal is to get all of the Lat, Lon coordinates of all users however im already stuck on how to get to Location collection if I dont know the document name where it is?

Is there some query that I can do to move straight inside to something like WhereEqualTo("Lat") ?

To begin I tried the following:

    db.collection("Users").get()
            .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                @Override
                public void onComplete(@NonNull Task<QuerySnapshot> task) {
                    if (task.isSuccessful()) {

                        List<String> listID = new ArrayList<>();
                        for (QueryDocumentSnapshot doc : task.getResult()) {

                            listID.add( doc.getId() );
                        }

However it returns me null list.

Thank you

Answer 1

You need to use a Collection Group Query , as follows:

db.collectionGroup("Location").get()
        .addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
            @Override
            public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
                // ...
            }
        });

This runs a query across all collections names Location in your database.