How to get snapshot from sub collection of main collection in firestore database real time in web application?

I need help in building web based real time chat application using firestore database. In website, I need list of users at left side who recently chatted and at right side, I need particular user chat conversation window. And I want to update left side user list with recent message whenever any of that user send or receive message. I do have firestore collection and its sub collection structure as explained below

ChatRooms (collection)
  ChatRoom1 (document)
    Messages (collection)
      Message1_1 (document)
      Message1_2 (document)
  ChatRoom2 (document)
    Messages (collection)
      Message2_1 (document)
      Message2_2 (document)

Now here, on my ChatRooms collection's onSnapShot event, I want to trigger snapshot if new message document is added in "Messages" collection. As I can get trigger when there is new document in "ChatRooms" collection but I am not able to get it from "Messages" collection which is actually sub collection. Will appreciate your help.