ndb.StructuredProperty not calling ndb.PolyModel subclass methods

Question

While storing ndb.Polymodel superclasses as an ndb.StructuredProperty, I was unable to access subclass methods; superclass methods were called instead and raising NotImplementedError's. Here is an abridged version of what I am trying to accomplish.

class Recipient(polymodel.PolyModel):
  name = ndb.StringProperty()

  def PrettyPrinting(self):
    raise NotImplementedError, 'Rawr'


class ShippingRecipient(Recipient):
  address_line_one = ndb.StringProperty()
  #there are other properties, but they aren't necessary here.

  def PrettyPrinting(self):
    return 'Hey, this should be called.'


class LocalRecipient(Recipient):
  distribution_location = ndb.StringProperty()
  #same deal, more attributes, but useless for this example.

  def PrettyPrinting(self):
    return 'Hey this should be called.'

class Shipment(ndb.Model):
  recipient = ndb.StructuredProperty(Recipient)

Now say that I have saved a shipment and stored a ShippingRecipient into the shipment's recipient field. In the datastore, the shipment recipient.class == ['Recipient', 'ShippingRecipient']. When I call:

shipment = Shipment.get_by_id('some_key')
shipment.recipient.PrettyPrinting()

The NotImplementedError is raised instead of the ShippingRecipient implementation of PrettyPrinting(...). I want the subclass method to be called when accessing the shipment's recipient field. Is there a way I can get to the subclass method instead? I know that saying the structured property is of type Recipient causes the superclass method to be called, but then maybe I don't fully understand why they would store the subclass in the recipient.class attribute.

Answer 1

I don't believe this can work. It will only store Recipient instances.
If you look at how PolyModel works, all variants are stored as the base class, in your example Recipient. It also stores the sub class names, when and entity it is retrieved from the data store, it recreates the specific subclass.

I really doubt they would have built this mechanism into StructuredProperty instantiation, and you have found this to be the case.