title: Google App Engine Query (not filter) for children of an entity
body: <p>Are the children of an entity available in a Query?</p>
  
  <p>Given:</p>
  
  <pre><code>class Factory(db.Model):
      """ Parent-kind """
      name = db.StringProperty()
  
  class Product(db.Model):
      """ Child kind, use Product(parent=factory) to make """
      @property
      def factory(self):
          return self.parent()
      serial = db.IntegerProperty()
  </code></pre>
  
  <p>Assume 500 factories have made 500 products for a total of 250,000 products.  Is there a way to form a resource-efficient query that will return just the 500 products made by one particular factory?  The ancestor method is a filter, so using e.g. Product.all().ancestor(factory_1) would require repeated calls to the datastore.</p>