Code

Chats.java

    public void loadChats() {

        final User_ currentUser = authenticateManager.getCurrentUser();
        if (currentUser != null) {

            //Fetch all the teachers associated to this student
            $FirebaseDatabase("/associations/{currentUser.getUserId()}/teachers").model(User_.class).setKeyIntoField("userId");
            if ($FirebaseDatabase.MultipleValues) {

                List<Chat_> chats = new LinkedList<>();
                List<User_> teachers = $inject("values");

                //Fetch all the chats associated
                $FirebaseDatabase("/chats").model(Chat_.class).setKeyIntoField("chatId")
                    .getPathReference().orderByChild("users/" + currentUser.getUserId() + "/userId")
                                       .equalTo(currentUser.getUserId());

                if ($FirebaseDatabase.MultipleValues) {

                    List<Chat_> existingChats = $inject("values");
                    Set<String> existingTeachersId = new HashSet<>();

                    for (Chat_ chat : existingChats) {
                        chats.add(chat);
                        existingTeachersId.addAll(chat.getUsers().keySet());
                    }

                    for (final User_ teacher : teachers) {

                        //Create "virtual" chats
                        if (!existingTeachersId.contains(teacher.getUserId())) {
                            Chat_ virtualChat = new Chat_();

                            virtualChat.setUsers(new HashMap<String, User_>() {{
                                put(currentUser.getUserId(), currentUser);
                                put(teacher.getUserId(), teacher);
                            }});

                            chats.add(virtualChat);
                        }
                    }

                    $ChatsLoaded(chats);

                    listenExistingChats(existingChats);

                }

                if ($FirebaseDatabase.NotExists) {

                    for (final User_ teacher : teachers) {

                        //Create "virtual" chats
                        Chat_ chat = new Chat_();

                        chat.setUsers(new HashMap<String, User_>() {{
                            put(currentUser.getUserId(), currentUser);
                            put(teacher.getUserId(), teacher);
                        }});

                        chats.add(chat);
                    }

                    $ChatsLoaded(chats);

                }

            }

            if ($FirebaseDatabase.NotExists) {
                $ChatsLoaded(new LinkedList<Chat_>());
            }

        }

    }

    protected void listenExistingChats(List<Chat_> existingChats) {

        for (Chat_ chat : existingChats) {
            listenExistingChat(chat);
        }

    }

    protected void listenExistingChat(Chat_ chat) {

        if (!chatsBeingListened.containsKey(chat.getChatId())) {

            FirebaseDatabaseActionHolder.FirebaseValueListener listener =
                $FirebaseDatabase("/messages/{chat.getChatId()}").keepSynced(true)
                                                                 .model(Message_.class)
                                                                 .setKeyIntoField("messageId")
                                                                 .listenChildValues();

            if ($FirebaseDatabase.ChildAdded) {
                Message_ message = $inject("value");
                $MessageAdded(message);
            }

            chatsBeingListened.put(chat.getChatId(), listener);

        }

    }