title: Query data in multiple collections
body: <p>I am developing a chat app using Firebase Firestore.
  I have a collection for room chat and another one for user info.
  How can I retrieve user data when I listen Room snapshot.</p>
  
  <p>This is my database structure:</p>
  
  <p><a href="https://i.stack.imgur.com/RVBV7.png" rel="noreferrer"><img src="https://i.stack.imgur.com/RVBV7.png" alt="enter image description here"></a></p>
  
  <p><a href="https://i.stack.imgur.com/MNpL3.png" rel="noreferrer"><img src="https://i.stack.imgur.com/MNpL3.png" alt="enter image description here"></a></p>
  
  <p>Please help</p>