title: BadValueError: Expected string, got Key when creating child entity
body: <p>Let me start by saying I'm really new to Python, app engine, and the datastore, so please be kind if I'm missing something obvious.</p>
  
  <p>I'm trying to set up some data in the datastore with ancestor relationships, sort of the same way as in the example guestbook application from Google:</p>
  
  <pre><code>class Guestbook(webapp2.RequestHandler):
    def post(self):
      greeting = Greeting(parent=guestbook_key)
  </code></pre>
  
  <p>I created this code as an experiment to see if I could get it to work:</p>
  
  <pre><code>class Subscriber(ndb.Model):
      user_nickname = ndb.StringProperty()
      user_id = ndb.StringProperty()
  
  class Music(ndb.Model):
      level = ndb.StringProperty() # top or sub
      parent = ndb.StringProperty() # only if sub
      title = ndb.StringProperty()
  
  class SetupHandler(webapp2.RequestHandler):
      def get(self):
          user = users.get_current_user()
          subscriber = Subscriber(id=user.user_id())
          subscriber.user_id = user.user_id()
          subscriber.user_nickname = user.nickname()
  
          haydn = Music(parent=subscriber.key)
          haydn.level = "top"
          haydn.title = "Haydn Trumpet Concerto"
          haydn.put()
  </code></pre>
  
  <p>It seems to me as though I'm trying to do basically the same thing.. but I get this error:</p>
  
  <blockquote>
    <p>File "... Application\datastoreexplore\main.py", line 41, in get
        haydn = Music(parent=subscriber.key) BadValueError: Expected string, got Key('Subscriber', '12169615523634875051')</p>
  </blockquote>
  
  <p>I've spent hours reading docs and searching through stackoverflow and elsewhere. I'm stumped. What am I doing wrong??</p>