how does a db object reference an ndb object in app engine?

gistfile1.txt

class NdbKeyProperty(db.Property):
    """Stores an ndb key in a db model."""
    def get_value_for_datastore(self, model_instance):
        value = super(NdbKeyProperty, self).get_value_for_datastore(
                model_instance)
        if value is None:
            return value
 
        # We were given an ndb key, and we want to store it as a db key
        return value.to_old_key()
 
    def make_value_from_datastore(self, value):
        if value is None:
            return None
        # From the db key, turn it into an ndb key
        return ndb.Key.from_old_key(value)
 
 
class _TargetModelNdb(ndb.Model):
    # A vanilla ndb model
    value = ndb.StringProperty(default='')
 
 
class _LinkByNdbKeyModel(db.Model):
    # A db model, that links to an ndb model via NdbKeyProperty
    target = NdbKeyProperty(indexed=True, default=None)
 
 
class NdbKeyPropertyTest(gae_model.GAEModelTestCase):
    def test_reading_default_value_returns_none(self):
        link = _LinkByNdbKeyModel()
        # This works
        self.assertIsNone(link.target)
 
    def test_reading_property_value_returns_an_ndb_key(self):
        target = _TargetModelNdb(value="test")
        target.put()
 
        link = _LinkByNdbKeyModel()
        link.target = target.key
        link.put()
        # These work
        self.assertIsInstance(link.target, ndb.Key)
        self.assertEqual(target.key, link.target)
    
        # Below will fail, as db queries get confused by the ndb key being passed in
        result = _LinkByNdbKeyModel.all().filter("target =", target.key).get()
 
# This is confusing because the opposite works. 
# (In other words, you can have an ndb object reference a db object, and filter by db key.)