title: Google App Engine NDB custom key id
body: <p>When I create an object with ndb's method put it creates the key automatically of the type Key(kind, id) where id is a number. All over the documentation it shows that you can use a string for the key's id but I couldn't find out how to do this automatically when an object is created. </p>
  
  <p>I have a User model and I was thinking to use the user's username (since its unique) as the key's id for faster retrieval. Is that even a good idea? Would I have any problems with the username since it's user submited (i'm validating the input)? </p>