I'm looking to implement a property class for appengine, very similar to the existing db.ReferenceProperty. I am implementing my own version because I want some other default return values. My question is, how do I make the property remember its returned value, so that the datastore query is only performed the first time the property is fetched? What I had is below, and it does not work. I read that the Property classes do not belong to the instances, but to the model definition, so I guess that the return value is not cached for each instance, but overwritten on the model every time. Where should I store this _resolved variable?

class PageProperty(db.Property):
  data_type = Page

  def get_value_for_datastore(self, model_instance):
    page = super(PageProperty, self).get_value_for_datastore(model_instance)        
    self._resolved = page
    return page.key().name()

  def make_value_from_datastore(self, value):
    if not hasattr(self, '_resolved'):
        self._resolved = Page.get_by_name(value)
    return self._resolved