如何使用分页在 Firebase 中实时降序？

Question

Im making query and trying to make an order by rating field. This is paginated list so im making partially queries.

But my query is not sorted.

   private void callRealOnlineUsersList(int page, String neededGender) {
    DatabaseReference mReference = realtimeReference.child("OnlineUsers/" + neededGender);
    Query query = mReference.limitToLast(page * TOTAL_ITEMS_TO_LOAD);
    query.orderByChild("rating");
    query.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            final List<OnlineUser> userList = new ArrayList<>();
            for (DataSnapshot postSnapshot : dataSnapshot.getChildren()) {
                OnlineUser user = postSnapshot.getValue(OnlineUser.class);
                userList.add(new OnlineUser(user.getUid(), user.getName(), user.getImage(), user.getGender(), user.getCountry(), user.getRating()));
            }
            EventBus.getDefault().post(new ListEvent(userList));
        }

Logs are

 D/testList: rating 2
            D/testList: rating 1
            D/testList: rating 25
            D/testList: rating 3
            D/testList: rating 1
            D/testList: rating 4
            D/testList: rating 1
            D/testList: rating 10
            D/testList: rating 2
            D/testList: rating 1
            D/testList: rating 25

Datasnapshot

Answer 1

So, you've combined 3 questions into 1:

• Why isn't my code sorting at all ?
• Why can't I sort in descending order?
• Given a query that works, how should I do pagination with Firebase RTDB on Android?

Good news, the later 2 of these have good existing answers on StackOverflow!

First, the way you're using Query here:

Query query = mReference.limitToLast(page * TOTAL_ITEMS_TO_LOAD);
query.orderByChild("rating");

isn't correct (and this is why you aren't getting sorted results at all ).

Instead try this:

Query query = mReference
                 .limitToLast(page * TOTAL_ITEMS_TO_LOAD)
                 .orderByChild("rating");

If you just call orderByChild , it returns a query that does that, but in your code you are ignoring the result.

To help make it clearer what is happening, you could also do this and it would work (but your current code doesn't save the result of the orderByChild call:

Query query = mReference.limitToLast(page * TOTAL_ITEMS_TO_LOAD);
query = query.orderByChild("rating");

Second, you can't sort by descending order in Firebase RTDB , so you should look at that question for possible solutions.

Finally, your current code will always be returning larger and larger numbers of results as page increases -- you don't always get just TOTAL_ITEMS_TO_LOAD results. A more complete SO question about pagination is here .