hibernate一对多关联问题

user table
单向一对多。。

    @OneToMany(fetch = FetchType.LAZY)
    @JoinColumn(name = "user_id")
    private List<Order> orders;

test

        User user = userDao.findOne(1L);
        Address address = new Address("1" , "2" , "3" ,"4");
        user.setAddresses(Lists.newArrayList(address));
        userDao.save(user);

打印结果：
select xxx from user xxx

insert into `tb_address` (`create_time`, `update_time`, `address`, `city`, `name`, `phone`, `id`) values (?, ?, ?, ?, ?, ?, ?)

update `tb_address` set `user_id`=null where `user_id`=?

update `tb_address` set `user_id`=? where `id`=?

！！！中间那句怎么产生的？
而且我插入数据，第一次插入数据库 user_id = 1
再执行几次后面插入的数据全部 user_id = null

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不能直接user.setAddresses(Lists.newArrayList(address));
需要get，再add，再save