对具有一对一关系的模型的更改未保存

我使用Django的OneToOneField建立了两个模型,如下所示: https://docs.djangoproject.com/en/2.0/topics/db/examples/one_to_one/

class Seats(models.Model):
    north = models.OneToOneField('User',on_delete=models.CASCADE,related_name='north', default=None, null=True)
    bridgetable = models.OneToOneField('BridgeTable',on_delete=models.CASCADE, default=None, null=True)

class BridgeTableManager(models.Manager):

    def create_deal(self):
        deal = construct_deal()
        table = self.create(deal=deal)
        s = Seats(bridgetable=table)
        s.save()
        return table

class BridgeTable(models.Model):
    deal = DealField(default=None,null=True)

当我运行此代码时,我可以成功地使关系正常工作

table = BridgeTable.objects.get(pk='1')
user = User.objects.get(username=username)
table.seats.north = user
table.seats.north.save()
print(table.seats.north)

print语句打印出坐在北方的球员的名字。但如果我再次尝试这样访问表:

table = BridgeTable.objects.get(pk='1')
print(table.seats.north)

我得到的是“None”,而不是用户名。是否有什么东西我遗漏了,比如我遗漏的一次扑救或我不理解的一些概念?谢谢