title: Firestore query subcollections
body: <p>I thought I read that you can query subcollections with the new Firebase Firestore, but I don't see any examples. For example I have my Firestore setup in the following way:</p>
  
  <ul>
  <li>Dances [collection]
  
  <ul>
  <li>danceName</li>
  <li>Songs [collection]  
  
  <ul>
  <li>songName</li>
  </ul></li>
  </ul></li>
  </ul>
  
  <p>How would I be able to query "Find all dances where songName == 'X'"</p>