title: Partly update App Engine entity
body: <p>I'm building a sync engine with App Engine and when I receive data from the client I want to store an object but I don't care if it already exists or not. It works nice today if I always send all properties from the client when updating. But I want...</p>
  
  <ul>
  <li>some internal properties not to be known by the client and still survive the update</li>
  <li>the client to be able to only send the changed values</li>
  <li>avoid fetching all objects before updating them as there can be quite few objects that need updates</li>
  </ul>
  
  <p><strong>Do I need to get each object and then update only the values I want to change and then update the object? Or is it possible to partly update entities without fetching them?</strong></p>