title: Google AppEngine: making complex queries
body: <p>I have a model like this:</p>
  
  <pre><code>class Person(db.Model):
       ...
       zipcode = db.StringProperty()
  
  class Book(db.Model):
       owner = db.ReferenceProperty(Person)
       date_created = db.DateTimeProperty(auto_add=True)
  </code></pre>
  
  <p>How can I get a list of all the books within certain zipcode that are sorted by <code>date_created</code> in descending order?</p>