title: google app engine's ndb: get an entity's id
body: <p>This looks simple, but I just didn't find how to get an entity's id from Google App Engine's ndb.</p>
  
  <pre><code>class Message(ndb.Model):
      name: ndb.StringProperty()
      ...
  </code></pre>
  
  <p>Create a message object:</p>
  
  <pre><code>message = Message(id=someId)
  message.name = someName
  message.put()
  </code></pre>
  
  <p>Later when I retrieve the entity:</p>
  
  <pre><code>message = Message.query(Message.name==someName).fetch(1)
  </code></pre>
  
  <p>Now how do I get the message's id? Thanks.</p>