ndb reference property

gistfile1.py

from google.appengine.ext import ndb

class ReferenceProperty(ndb.KeyProperty):
    def _validate(self, value):
        if not isinstance(value, ndb.Model):
            raise TypeError('expected an ndb.Model, got %s' % repr(value))

    def _to_base_type(self, value):
        return value.key

    def _from_base_type(self, value):
        return value.get()