firebase - How to structure NoSQL messages to get unreads by 1 query?

Question

Say I have NoSQL structure as follows:

messages  
  chat_id (known)  
    message_id (generated automatically)  
      {author, timestamp, content}

I also have users/ branch where I can see last logins of both users involved.
I want to get the number of unread messages for a given user by 1 query.

How would you implement such a task?

Answer 1

Waitting for answers