title: Filtering app engine data from datastore dynamically
body: <p>My app-engine data model is defined as:</p>
  
  <pre><code>class Event(db.Model):
      title = db.LinkProperty();
      refresh_interval = db.IntegerProperty(); 
  
  class EventSchedule(db.Model):
      event = db.ReferenceProperty(Event)     # referencing the event
      refresh_date = db.DateTimeProperty();   # date &amp; time of last successful refresh 
  </code></pre>
  
  <p>I'd like to fetch all EventSchedule items based on the criteria of: </p>
  
  <pre><code>event_schedule.refresh_date + event.refresh_interval &gt;= now
  </code></pre>
  
  <p>In SQL-land, it would look something like:
      DATE_ADD(event_schedule.refresh_date, event.refreshInterval) >= now()</p>
  
  <p>Is this kind of join operation feasible with the data store? </p>