class MyModel(db.Model):
	ref_id = db.IntegerProperty()

	def _get_ref_key(self):
		if self.ref_id is None:
			return None
		elif self.ref_id == -1:
			return self.key()
		else:
			return db.Key.from_path(self.kind(), self.ref_id)
	ref_key = property(_get_ref_key)