title: Firestore cloud listener, not working on collection inside collection
body: <p>I am setting up a program which uses the Admin Sdk from Firebase, it needs to be able to update on any changes or documents added on a collection. The problem is that it doesn't update when it is a collection/document/collection/document/collection.</p>
  
  <p>my code is this. (doesn't work)</p>
  
  <pre><code>import time
  import firebase_admin
  from firebase_admin import credentials
  from google.cloud import firestore
  def on_snapshot(doc_snapshot, changes, read_time):
      for doc in doc_snapshot:
          print(u'Received document snapshot: {}'.format(doc.id))
  
  cred = credentials.Certificate('cred.json')
  default_app=firebase_admin.initialize_app(cred)
  db = firestore.Client()
  doc_ref = db.collection('company').document('main').collection('restaurant').document('info').collection('orders')
  doc_watch = doc_ref.on_snapshot(on_snapshot)
  #for this example i will use a while (it works too)
  while True:
     time.sleep(1)
     print ('processing...')
  </code></pre>
  
  <p>but it works with this doc_ref </p>
  
  <pre><code>doc_ref = db.collection('orders')
  </code></pre>
  
  <p>and also like this</p>
  
  <pre><code>db.collection('company').document('main').collection('restaurant')
  </code></pre>
  
  <p>am i doing something wrong?</p>