单向N-1

情景: 合租，假设你要租房子，多个人住同一个地方，对于你来说，你只需要关心我今天晚上要住到哪里，而不需要关心。某一个地址住了哪些人，（其实还是应该关心一下的）。
表结构如下

| Field    | Type         | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+-------+
| id       | varchar(50)  | NO   | PRI | NULL    |       |
| location | varchar(100) | NO   |     | NULL    |       |
+----------+--------------+------+-----+---------+-------+

+------------+-------------+------+-----+---------+-------+
| Field      | Type        | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| id         | varchar(50) | NO   | PRI | NULL    |       |
| name       | varchar(50) | NO   |     | NULL    |       |
| age        | int(5)      | YES  |     | NULL    |       |
| address_id | varchar(50) | YES  | MUL | NULL    |       |
+------------+-------------+------+-----+---------+-------+

无链接表的单向N-1

---

1. 在N一端的实体增加@ManyToOne 修饰的 关联实体
2. 在N一端增加外键列，实体类中用@JoinColumn修饰关联实体。

@Entity
@Table(name="tb_person")
public class Person extends AbstractEntity{
      // id 我在AbstractEntity中声明了
      // 需要将AbstractEntity 添加@MappedSuperclass
      @Column(name="name")
      private String name;

      @Column(name="age")
      private Integer age;

      //用CasecadeType=All 主要方便的是在 Person 更新，删除，添加的时候自动把address持久化
      @ManyToOne(cascade=CascadeType.ALL)
      @JoinColumn(name="address_id")
      private Address address;

      //省略set.get方法
}

因为不需要从address获取person 所以 address 只需要自己本身的属性就行。

 @Entity
@Table(name="tb_address")
public class Address extends AbstractEntity{

    @Column(name="location")
    private String addressDetail;
    
    public String getAddressDetail() {
        return addressDetail;
    }

    public void setAddressDetail(String addressDetail) {
        this.addressDetail = addressDetail;
    }
}

测试用例

  @Test
  @Transactional
  @Rollback(value=false)
   public void testEntity(){
        Session session = sessionFactory.getCurrentSession();
        Person p = new Person();        
        Address address = new Address();
        address.setAddressDetail("ZheJiang");
        p.setName("WhosYourdaddy");
        p.setAge(89);
        
        p.setAddress(address);

        session.save(p);
    }

测试结果

  +--------------------+----------+                  
| id                 | location |
+--------------------+----------+
| 170413181716030431 | ZheJiang |
+--------------------+----------+

  +--------------------+---------------+------+--------------------+
| id                 | name          | age  | address_id         |
+--------------------+---------------+------+--------------------+
| 170413181716014566 | WhosYourdaddy |   89 | 170413181716030431 |
+--------------------+---------------+------+--------------------+

我们再来看一下sql输出，

第一条
/* get current state com.statestr.verify.entity.Address */ 
select address_.id, address_.location as location2_0_ from tb_address address_ where address_.id=?
HibernateLog --> 18:17:16 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [1] as [VARCHAR] - [170413181716030431]
第二条
/* insert com.statestr.verify.entity.Address*/ 
insert into tb_address (location, id) values (?, ?)
HibernateLog --> 18:17:16 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [1] as [VARCHAR] - [ZheJiang]
HibernateLog --> 18:17:16 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [2] as [VARCHAR] - [170413181716030431]
第三条
insert com.statestr.verify.entity.Person*/ insert into tb_person (address_id, age, name, id) values (?, ?, ?, ?)
HibernateLog --> 18:17:16 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [1] as [VARCHAR] - [170413181716030431]
HibernateLog --> 18:17:16 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [2] as [INTEGER] - [89]
HibernateLog --> 18:17:16 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [3] as [VARCHAR] - [WhosYourdaddy]
HibernateLog --> 18:17:16 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [4] as [VARCHAR] - [170413181716014566]

总共执行了3条语句。第一条是查询是否数据库已经有了identity 相等的address。如果没有那就需要insert address.

那么如果数据库已经存在该条数据库呢？

Address address = (Address)session.get(Address.class, "170413181716030431");
// 替换
Address address = new Address();
address.setAddressDetail("ZheJiang");

第一条
 select address0_.id as id1_0_0_, address0_.location as location2_0_0_ from tb_address address0_ where address0_.id=?
HibernateLog --> 19:08:43 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [1] as [VARCHAR] - [170413181716030431]
第二条
/* insert com.statestr.verify.entity.Person*/ 
insert into tb_person (address_id, age, name, id) values (?, ?, ?, ?)
HibernateLog --> 19:08:43 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [1] as [VARCHAR] - [170413181716030431]
HibernateLog --> 19:08:43 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [2] as [INTEGER] - [89]
HibernateLog --> 19:08:43 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [3] as [VARCHAR] - [WhosYourdaddy]
HibernateLog --> 19:08:43 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [4] as [VARCHAR] - [170413190843156966]

只执行了2条语句，因为它判断addres已经在数据库里面，而且没有改变其内容。hibernate 会跟踪每个持久化状态对象的改动。（可以参阅 http://www.bbsmax.com/A/amd03O6X5g/ ，我发现在我这个4.3.11.Final，其实所有持久化对象都存储在Session的 persistenceContext，变量中。持久化对象发生改变时，他下面的确改变了，但是没有找到保存之前状态的地方。）这里它发现对象并没有被改变。那如果对象发生改变了呢？

Address address = (Address)session.get(Address.class, "170413181716030431");
address.setAddressDetail("QingDao");
// 替换
Address address = (Address)session.get(Address.class, "170413181716030431");

// 第一条 查找数据库
 select address0_.id as id1_0_0_, address0_.location as location2_0_0_ from tb_address address0_ where address0_.id=?
HibernateLog --> 19:22:02 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [1] as [VARCHAR] - [170413181716030431]
// 第二条 添加person到数据库
/* insert com.statestr.verify.entity.Person*/ insert into tb_person (address_id, age, name, id) values (?, ?, ?, ?)
HibernateLog --> 19:30:50 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [1] as [VARCHAR] - [170413181716030431]
HibernateLog --> 19:30:50 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [2] as [INTEGER] - [89]
HibernateLog --> 19:30:50 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [3] as [VARCHAR] - [WhosYourdaddy]
HibernateLog --> 19:30:50 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [4] as [VARCHAR] - [170413192101018079]
//第三条 更新address，hibernate session 检测到持久类已经发生改变
update com.statestr.verify.entity.Address */ update tb_address set location=? where id=?
HibernateLog --> 19:30:50 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [1] as [VARCHAR] - [QingDao]
HibernateLog --> 19:30:50 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [2] as [VARCHAR] - [170413181716030431]

还要注意一下调用的顺序，稍微有点偏题了...

有链接表的单向N-1

---

绝大多数单向N-1，都是用基于外键的。但hibernate 也支持了有连接表的N-1关系。（不常用，留个印象就行）
有连接表的单向N-1就需要用到@JoinTable，他主要用来映射底层数据库中间表。主要属性有

属性	是否必须	说明
name	否	（重要）中间表的表名
catalog	否	设置中间表放入制定的catalog
schema	否	设置中间表放入制定的schema
targetEntity	否	设定关联实体的类名。一般不需要设置，hibernate自动会获取
indexes	否	为连接表设置多个索引
joinColumns	否	（重要）接收JoinColumn，这边设置的是JoinTable表对应当前实体的外键列
inverseJoinColumns	否	（重要）接收JoinColumn，这边设置的是JoinTable表对应关联实体的外键列
uniqueConstraints	否	增加唯一性约束

（安利一个好的网站 table Generate,可以生成markdow的表格）

好接下来我们来看例子:

表结构

mysql> describe tb_person_addr_map;
+------------+-------------+------+-----+---------+-------+
| Field      | Type        | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| person_id  | varchar(50) | NO   | MUL | NULL    |       |
| address_id | varchar(50) | NO   | MUL | NULL    |       |
+------------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> describe tb_person;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | varchar(50) | NO   | PRI | NULL    |       |
| name  | varchar(50) | NO   |     | NULL    |       |
| age   | int(5)      | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.02 sec)

mysql> describe tb_address;
+----------+--------------+------+-----+---------+-------+
| Field    | Type         | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+-------+
| id       | varchar(50)  | NO   | PRI | NULL    |       |
| location | varchar(100) | NO   |     | NULL    |       |
+----------+--------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

这边只需要该Person实体

@Entity
@Table(name="tb_person")
public class Person extends AbstractEntity{
    
    
    @Column(name="name")
    private String name;
    
    
    @Column(name="age")
    private Integer age;
    
    @ManyToOne(cascade=CascadeType.ALL)
    @JoinTable(name="tb_person_addr_map",joinColumns=@JoinColumn(name="person_id",unique=true),
                 inverseJoinColumns=@JoinColumn(name="address_id"))
    private Address address;
        // 省略set、get 方法
}

因为是N-1关联，那么Person只能有一个Address，所以添加了unique=true
我们来看sql输出:

/* get current state com.statestr.verify.entity.Address */
select address_.id, address_.location as location2_0_ from tb_address address_ where address_.id=?
HibernateLog --> 16:50:50 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [1] as [VARCHAR] - [170414165050149527]
 /* insert com.statestr.verify.entity.Address*/ 
insert into tb_address (location, id) values (?, ?)
HibernateLog --> 16:50:50 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [1] as [VARCHAR] - [ZheJiang]
HibernateLog --> 16:50:50 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [2] as [VARCHAR] - [170414165050149527]
/* insert com.statestr.verify.entity.Person*/ 
insert into tb_person (age, name, id) values (?, ?, ?)
HibernateLog --> 16:50:50 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [1] as [INTEGER] - [89]
HibernateLog --> 16:50:50 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [2] as [VARCHAR] - [WhosYourdaddy]
HibernateLog --> 16:50:50 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [3] as [VARCHAR] - [170414165050149349]
/* insert com.statestr.verify.entity.Person*/ 
insert into tb_person_addr_map (address_id, person_id) values (?, ?)
HibernateLog --> 16:50:50 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [1] as [VARCHAR] - [170414165050149527]
HibernateLog --> 16:50:50 TRACE org.hibernate.type.descriptor.sql.BasicBinder - binding parameter [2] as [VARCHAR] - [170414165050149349]

一共执行了4条语句。可以看到这边最后才插入到中间表中。