关于hibernate中双向外键关联one-to-one的property-ref=的有关问题

关于hibernate中双向外键关联one-to-one的property-ref=的问题

    大家都知道hibernate中的one-to-one映射主要有两种策略，（1）一对一主键关联（单向和双向）。（2）一对一外键映射（单项和双向）。本文主要讲解一下，一对一外键映射中的双向问题，在此前先通过一个实例了解。

person和idCard,是一种一对一的关系，其中

t_person表
id        name       idCard(unique)
1         张三
2        王五       1

其中王五是没有idcard，这也符合现实中的，有些人是没有身份证的。
t_idCard表
id         cardNo
1         11111111111111

实体类：
IdCard

<span style="font-family:SimHei;font-size:18px;color:#000000;">package com.bjpowernode.hibernate; 

public class IdCard { 

private int id; 

private String cardNo; 

private Person person; 

public Person getPerson() { 
return person; 
} 

public void setPerson(Person person) { 
this.person = person; 
} 

public int getId() { 
return id; 
} 

public void setId(int id) { 
this.id = id; 
} 

public String getCardNo() { 
return cardNo; 
} 

public void setCardNo(String cardNo) { 
this.cardNo = cardNo; 
} 

} 
</span>

Person

<span style="font-family:SimHei;font-size:18px;color:#000000;">package com.bjpowernode.hibernate; 

public class Person { 

private int id; 

private String name; 

private IdCard idCard; 

public int getId() { 
return id; 
} 

public void setId(int id) { 
this.id = id; 
} 

public String getName() { 
return name; 
} 

public void setName(String name) { 
this.name = name; 
} 

public IdCard getIdCard() { 
return idCard; 
} 

public void setIdCard(IdCard idCard) { 
this.idCard = idCard; 
} 
} 
</span>

（3）配置文件
IdCard的：

<span style="font-family:SimHei;font-size:18px;color:#000000;"> <?xml version="1.0"?> 
<!DOCTYPE hibernate-mapping PUBLIC 
"-//Hibernate/Hibernate Mapping DTD 3.0//EN" 
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 
<hibernate-mapping package="com.bjpowernode.hibernate"> 
<class name="IdCard" table="t_idCard"> 
<id name="id"> 
<generator class="native"/> 
</id> 
<property name="cardNo"/> 
<one-to-one name="person"  class="Person"  /> 
</class> 
</hibernate-mapping> 
</span>

Person的：

<span style="font-family:SimHei;"><span style="font-size:18px;color:#000000;"><?xml version="1.0"?> 
<!DOCTYPE hibernate-mapping PUBLIC 
"-//Hibernate/Hibernate Mapping DTD 3.0//EN" 
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 
<hibernate-mapping package="com.bjpowernode.hibernate"> 
<class name="Person" table="t_person"> 
<id name="id"> 
<generator class="native"/> 
</id> 
<property name="name"/> 
<many-to-one name="idCard" cascade="all" class="IdCard" 
unique="true" column="card_ID" /> 
</class> 
</hibernate-mapping> 

</span><span style="font-size:18px;color:#000000;">（3）</span></span><span style="color:#000000;"><span style="font-family:SimHei;font-size:18px;"> 
向t_person中插入数据： </span></span>

<span style="font-family:SimHei;font-size:18px;color:#000000;">import org.hibernate.Session; 

import junit.framework.TestCase; 

public class One2OneTest extends TestCase { 

public void testSave1() { 
Session session = null; 
try { 
session = HibernateUtils.getSession(); 
session.beginTransaction(); 

Person person = new Person(); 
person.setName("张三"); 

session.save(person); 
session.getTransaction().commit(); 
}catch(Exception e) { 
e.printStackTrace(); 
session.getTransaction().rollback(); 
}finally { 
HibernateUtils.closeSession(session); 
} </span>

结果如下：

<span style="font-family:SimHei;font-size:18px;color:#000000;">mysql> select * from t_person; </span>

+----+------+---------+
| id | name | card_ID |
+----+------+---------+
| 1 | 张三 | NULL |
+----+------+---------+
1 row in set (0.00 sec)

<span style="font-family:SimHei;font-size:18px;color:#000000;">mysql> select * from t_idcard; </span>

Empty set (0.00 sec)

在插入数据：

<span style="font-family:SimHei;font-size:18px;color:#000000;">public void testSave2() { 
Session session = null; 
try { 
session = HibernateUtils.getSession(); 
session.beginTransaction(); 

IdCard idCard = new IdCard(); 
idCard.setCardNo("1111111111"); 
session.save(idCard); 

Person person = new Person(); 
person.setName("王五"); 
//建立关联 
person.setIdCard(idCard); 

session.save(person); 

session.getTransaction().commit(); 
}catch(Exception e) { 
e.printStackTrace(); 
session.getTransaction().rollback(); 
}finally { 
HibernateUtils.closeSession(session); 
} 
} </span>

数据库中的结果如下：

<span style="font-family:SimHei;font-size:18px;color:#000000;">mysql> select * from t_person; </span>

+----+------+---------+
| id | name | card_ID |
+----+------+---------+
| 1 | 张三 | NULL |
| 2 | 王五 | 1 |
+----+------+---------+
2 rows in set (0.00 sec)

<span style="font-family:SimHei;font-size:18px;color:#000000;">mysql> select * from t_idcard; </span>

+----+------------+
| id | cardNo |
+----+------------+
| 1 | 1111111111 |
+----+------------+
1 row in set (0.00 sec)
（4）加载数据，这样的话就可以从person的一端加载到idCard，
如下：

<span style="font-family:SimHei;font-size:18px;color:#000000;">public void testLoad1() { 
Session session = null; 
try { 
session = HibernateUtils.getSession(); 
session.beginTransaction(); 
Person person = (Person)session.load(Person.class, 2); 
System.out.println("person.name=" + person.getName()); 
System.out.println("person.cardNo=" +                   person.getIdCard().getCardNo()); 
session.getTransaction().commit(); 
}catch(Exception e) { 
e.printStackTrace(); 
session.getTransaction().rollback(); 
}finally { 
HibernateUtils.closeSession(session); 
} 
} </span>

结果如下：
    Hibernate: select person0_.id as id0_0_, person0_.name as name0_0_, person0_.card_ID as card3_0_0_ from t_person person0_ where person0_.id=?
person.name=王五
    Hibernate: select idcard0_.id as id1_1_, idcard0_.cardNo as cardNo1_1_, person1_.id as id0_0_, person1_.name as name0_0_, person1_.card_ID as card3_0_0_ from t_idCard idcard0_ left outer join t_person person1_ on idcard0_.id=person1_.id where idcard0_.id=?
    person.cardNo=1111111111

这样的就找到了person对应的idcard，那能不能有idCard找到person呢？

<span style="font-family:SimHei;font-size:18px;color:#000000;">public void testLoad2() { 
Session session = null; 
try { 
session = HibernateUtils.getSession(); 
session.beginTransaction(); 
//                      
IdCard idCard = (IdCard)session.load(IdCard.class, 1); 
System.out.println("idCard.cardNo=" + idCard.getCardNo()); 
System.out.println("idCard.person.name=" + idCard.getPerson().getName()); 
session.getTransaction().commit(); 
}catch(Exception e) { 
e.printStackTrace(); 
session.getTransaction().rollback(); 
}finally { 
HibernateUtils.closeSession(session); 
} 
} </span>

结果如下：
Hibernate: select idcard0_.id as id1_1_, idcard0_.cardNo as cardNo1_1_, person1_.id as id0_0_, person1_.name as name0_0_, person1_.card_ID as card3_0_0_ from t_idCard idcard0_ left outer join t_person person1_ on idcard0_.id=person1_.id where idcard0_.id=?
idCard.cardNo=1111111111
idCard.person.name=张三
结果对吗？
肯定不对，idCard.cardNo=1111111111这是王五的idCard，怎么查出来张三的呢？原因在于：
idCard的配置文件问题：
应该在idCard的配置文件的<one-to-one name="person" class="Person" />
修改为<one-to-one name="person" class="Person" property-ref="idCard"/>
因为：如果不修改则idCard会根据自己的id和person中的id比较（因为one-to-one是通过id查找到的），这样是不符合要求的，因为我们t_idcard中的id和t_person中 Card_ID相比较，这样的话可以通过
property-ref="idCard" 的设置找到t_person表中Card_ID和它作比较找到我们要找的数据。

总结：

property-ref: 指定关联类的属性名，这个属性将会和本类的主键相对应。如果没有指定，会使用对方关联类的主键。

property-ref，不是数据库表中的字段名,而是定义的java类中的属性名,一定要注意。

关于hibernate中双向外键关联one-to-one的property-ref=的有关问题

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