Firestore - Merging two queries locally

Question

Since there is no logical OR operator in Firestore, I am trying to merge 2 separate queries locally.

Now I wonder how I can keep up the proper order of the results. When I run 2 queries independently, I can't oder the results specificly (at least not the order in which I get the results from Firestore with the orderBy method).

My idea was to put the 2nd query inside the onSuccessListener of the 1st query. Is this a bad idea performance wise?

public void loadNotes(View v) {
    collectionRef.whereLessThan("priority", 2)
            .orderBy("priority")
            .get()
            .addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
                @Override
                public void onSuccess(QuerySnapshot queryDocumentSnapshots) {

                    for (QueryDocumentSnapshot documentSnapshot : queryDocumentSnapshots) {
                        Note note = documentSnapshot.toObject(Note.class);
                        //adding the results to a List
                    }

                    collectionRef.whereGreaterThan("priority", 2)
                            .orderBy("priority")
                            .get()
                            .addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
                                @Override
                                public void onSuccess(QuerySnapshot queryDocumentSnapshots) {

                                    for (QueryDocumentSnapshot documentSnapshot : queryDocumentSnapshots) {
                                        Note note = documentSnapshot.toObject(Note.class);
                                        //adding the results to a List
                                    }
                                }
                            });
                }
            });
}

Answer 1

To merge 2 separate queries locally, I recommend you to use Tasks.whenAllSuccess() method. You can achieve this, using the following lines of code:

FirebaseFirestore rootRef = FirebaseFirestore.getInstance();
Query firstQuery = rootRef...
Query secondQuery = rootRef...

Task firstTask = firstQuery.get();
Task secondTask = secondQuery.get();

Task combinedTask = Tasks.whenAllSuccess(firstTask, secondTask).addOnSuccessListener(new OnSuccessListener<List<Object>>() {
    @Override
    public void onSuccess(List<Object> list) {
         //Do what you need to do with your list
    }
});

As you can see, when overriding the onSuccess() method the result is a list of objects which has the exact order of the tasks that were passed as arguments into the whenAllSuccess() method.

There is also another approach and that would be to use Tasks.continueWith() method. But according to the use-case of your app, you can use eiter whenAllSuccess() method or continueWith() method. Please see here the official documentation.