title: why model has the key_name don't has the key().id() on google-app-engine
body: <p>if i use this :</p>
  
  <pre><code>class A(db.Model):
      a=db.StringProperty()
  
  class demo(BaseRequestHandler):
      def get(self):
          a=A()
          a.a='sss'
          a.put()
          raise Exception(a.key().id())
  </code></pre>
  
  <p>i can get the a.key().id() is 961</p>
  
  <p>but if i add key_name="aaa" , the a.key().id() will be None :</p>
  
  <pre><code>class A(db.Model):
      a=db.StringProperty()
  
  class demo(BaseRequestHandler):
      def get(self):
          a=A(key_name="aaa")
          a.a='sss'
          a.put()
          raise Exception(a.key().id())
  </code></pre>
  
  <p>so how can i get the key().id() when i set a key_name</p>
  
  <p>thanks</p>