title: Is it possible to query multiple document sub-collections in Cloud Firestore?
body: <p>I'm planning the structure of my DB for a project using Cloud Firestore, and I'm currently trying to figure out the best approach to storing data and associating it to users. The specifics are as follows:</p>
  
  <p>I have a collection of user documents at the root of my DB, and I want users to be able to make posts that are accessible both by looking at a user profile page which displays posts by that user, and in aggregate by looking at a page displaying posts by all users.</p>
  
  <p>Storing a user's posts in a sub-collection of the user document would make it very easy to fetch a user's posts for their profile, but is it possible to write an elegant query that merges every user's post sub-collections into one result set?</p>
  
  <p>Or would a better approach to be to have another collection in the root of my called posts, and have a field on each post containing the user's UID or a denormalized copy of the user document?</p>