title: Filtering documents basing on the documents in their subcollections
body: <p>Consider the following Firestore structure:</p>
  
  <ul>
  <li>Collection <code>people</code></li>
  <li>Every document in the collection has a subcollection <code>pets</code></li>
  <li>Subcollection <code>pets</code> has multiple documents with pet data.</li>
  </ul>
  
  <p>Now, I want to query for people whose pets satisfy certain conditions. Such as, people who have more than two pets, or who own a pet whose name is "ABC".</p>
  
  <p>Is this possible?</p>