title: Endpoints datastore - query with and without ancestors
body: <p>Similar to the <a href="http://endpoints-proto-datastore.appspot.com/examples/keys_with_ancestors.html" rel="nofollow">keys_with_ancestors.html</a> example I need to query a model by it's parent.
  But in addition on a different API request I need to query the same model without the parent restriction.</p>
  
  <p>The use case is that I have Tasks associated with Users, I need one API call listing the tasks of a given user and one listing all tasks.
  User need to be parent of Task to allow transactions.</p>
  
  <p>If I follow the keys_with_ancestors example I can't manage to to create the 'list all tasks' api - since the query building is done on the Task object and looks for the parent from the request.</p>
  
  <p>I can give up the parent and have a user_email property in the task and just have it as an optional query parameter - but this seems a bad solution.   </p>
  
  <p>Any better idea?</p>