title: Firebase Firestore subcollection of documents from document
body: <p>In my collection's SnapshotListener, I have am looping through the documents with:</p>
  
  <pre><code>for (DocumentChange doc : queryDocumentSnapshots.getDocumentChanges()) {
      if (doc.getType() == DocumentChange.Type.ADDED) {
          String somestr = doc.getDocument.getString("someField");
          // retrieve collection here
      }
  }
  </code></pre>
  
  <p>And within each document in the for loop, let's call it <code>doc</code>, I have another collection of documents on Cloud Firestore.  I would like to retrieve all documents (each of which just has a single String field) within the subcollection inside <code>doc</code>.  I'm able to do a <code>doc.getDocument</code> to get <code>doc</code> itself, which I am using to get a string field, but I don't know how to get the documents in its subcollection.</p>