Firestore chat app using Firebase-UI (Android)

Question

I am trying to create a chat using the Firebase-UI library for Cloud Firestore. This github repository contains the relevant code which I am using. The problem comes with the order of the query.

See that the query is specified as:

Query sChatQuery = sChatCollection.orderBy("timestamp").limit(50);

However, I am getting the oldest 50 messages, instead of the newest ones, in the correct order (from old to new). On the other hand, I can use the following query:

Query sChatQuery = sChatCollection.orderBy("timestamp", Query.Direction.DESCENDING).limit(50);

and this retrieves the 50 newest messages, but in the wrong order (newest on top, oldest on bottom). Hence I do not know how to get this right.

I could locally reverse again the result from the query, but I cannot figure out how to do that (I have already gone through the FirebaseUI library with no luck).

EDIT

I have my chat subcollection for each event in my events collection:

events/event_doc/chat/chat_doc

And I would like to get the messages as follows:

Message 1
Message 2
Message 3
Message 4
...

With the default order I get the messages like shown above, but the limit instructions cuts the snapshots from 1 to 50, and the Message 51 (the newest) is not being retrieved.

Answer 1

To solve this, you can use the following query:

Query sChatQuery = sChatCollection
    .orderBy("timestamp", Query.Direction.ASCENDING)
    .whereGreaterThanOrEqualTo("timestamp', desiredTime)
    .limit(50);

If you are using a RecyclerView to display data, the simplest way would be to use the following code:

LinearLayoutManager layoutManager = new LinearLayoutManager(getActivity());
layoutManager.setReverseLayout(true);
layoutManager.setStackFromEnd(true);
recyclerView.setLayoutManager(layoutManager);

This approch will reverse you the order as you want.

Another approach would be to create your own adapter that extends FirebaseListAdapter and override getItem() method like this:

@Override
public HashMap getItem(int pos) {
    return super.getItem(getCount() - 1 - pos);
}

Another approach would be to get the data from the database, add it to a Collection, a List would be a good solution and then use Collections.reverse(yourList);.