@ndb.transactional(xg=True)
def transfer_payments(key1, key2, amount):
    [p1, p2] = ndb.get_multi([key1, key2])
    p1.payments += amount
    p2.payments -= amount
    ndb.put_multi([p1, p2])

class Person(ndb.Model)
    payments = ndb.IntegerProperty(default=0)
    foo = ndb.BooleanProperty()

    def receive_payment(self, other, amount):
        transfer_payments(self.key, other.key, amount) # this is a transaction
        # self now has old data
        self.foo = True
        self.put()