Advice for Chat schema design

Working with Data
1

Hi, in my application I’m developing I need to allow for chats between any 2 users. I have the following schema in mind for a Messages collection:

{
  _id: ObjectId(),
  userID1: "userA_ID",
  userID2: "userB_ID",
  sender: "one of the users",
  message: "some text",
  timestamp: time
}

userID1 will always be the first user to start the chat, I’ll save this for every Chat room consisting of 2 people. (EDIT: Should I use lexicographical ordering instead, with userID1 coming before userID2?)

I’ll create an index on the timestamp field so I can sort it in reverse, and then I can do the following to get the data every time the chat room is loaded (with pagination as you scroll up):

db.find({ userID1, userID2 }).sort({ "timestamp" : -1 })
  .skip(offset).limit(limit)

And this I think should give me my intended behavior.

  1. Does this make sense? Is there anything I’m missing out or have overlooked?
  2. Is this the correct practice, storing ALL user messages in one big Messages collection? Of course I’ll be implementing all necessary security/privacy protection but the fact still remains that all these messages are stored in one large collection. Is this a concern from a security/privacy/logical standpoint?

Appreciate any advice, thanks.