title: Firestore cannot get path onsnapshot
body: <p>I am listening to changes on the database for <a href="https://firebase.google.com/docs/firestore/query-data/queries#collection-group-query" rel="nofollow noreferrer">collection_group</a> 
  I cannot access ref (which has path) of a DocumentSnapshot I keep getting the error:</p>
  
  <pre><code>AttributeError: 'DocumentSnapshot' object has no attribute 'ref'
  </code></pre>
  
  <p>Here is my code:</p>
  
  <pre><code>doc_ref = firestore_db.collection_group(u'collection_name')
  doc_ref.on_snapshot(self.__get_snapshot(args))
  </code></pre>
  
  <p>This is my __get_snapshot method:</p>
  
  <pre><code>def __get_snapshot(self, args):
      def on_snapshot(doc_snapshot, changes, read_time):
          for doc in doc_snapshot: #crashes
              print(u'Received document snapshot: {}'.format(doc.ref))
          for change in changes:
              if(change.type.name == "MODIFIED"):
                  print(change.document.ref) #crashes
                  print(change.document.get("field"))#this works fine
  return on_snapshot
  </code></pre>