title: Google Appengine Datastore I want to add primary_key
body: <p>I have a ndb model and I use Python + google appengine. I want to add primary key to my first field "name". Forexample I added a name "Ada", after that if I want to add again "Ada", it mustnt add, it gives me an error. I can do it easily with django but I couldnt figure out with google appengine. Can you show me a solution way please. Here is my codes:</p>
  
  <pre><code>class User(ndb.Model):
      username = ndb.StringProperty(indexed=False)
      created_date = ndb.DateTimeProperty(auto_now_add=True)
      updated_date = ndb.DateTimeProperty(auto_now_add=True)
  </code></pre>