hibernate一对一外键关联映射

一对一外键关联映射：在关系模型上多的一端持有一的一端一个引用，在关系模型上多的一端持有一的一端的外键。

对象模型:

Person: personId(Integer),name(String),age(Integer),idCard(IdCard)

IdCard:id(Integer),usefulLife(Date),person(Person)

关系模型：

person表



idCard表



Person.hbm.xml

<class name="bean.Person" table="person" catalog="addressbooksample">
<id name="personId" type="java.lang.Integer">
<column name="personId" />
<generator class="native" />
</id>
<property name="name" type="java.lang.String">
<column name="name" length="20" />
</property>
<property name="age" type="java.lang.Integer">
<column name="age" />
</property>
<one-to-one name="idCard" cascade="save-update" property-ref="person"/>
</class>

IdCard.hbm.xml

<class name="bean.IdCard" table="id_card" catalog="addressbooksample">
<id name="id" type="java.lang.Integer">
<column name="id" />
<generator class="native" />
</id>
<property name="userfulLife" type="java.util.Date">
<column name="userful_life" length="0" />
</property>
<many-to-one name="person" column="person_Id" unique="true" lazy="false"/>
</class>

测试

-----------------------------存储--------------------------------------------------------

Person person=new Person();
person.setAge(60);
person.setName("郭富城");

IdCard idCard=new IdCard();
idCard.setUserfulLife(new Date());
idCard.setPerson(person); //注释掉此句，在idCard表personId字段将为null
person.setIdCard(idCard);
personDao.save(person);//因在Person.hbm.xml中元素<one-to-one name=”idCard”>中配置了cascade=”true”,保存person对象的同时会保存关联对象

-----------------------------检索------------------------------------

Person person=personDao.get(24);
System.out.println(person.getIdCard().getUserfulLife());

去掉Person.hbm.xml中的

<one-to-one name="idCard" cascade="save-update"
property-ref="person"/>
只发出 select person0_.personId as personId0_0_, person0_.name as name0_0_, person0_.age as age0_0_ from addressbooksample.person person0_ where person0_.personId=?

将只在person表中查找，所以输出语句将发出空指针异常

将<one-to-one>改成<one-to-one name="idCard" cascade="save-update"/>
select person0_.personId as personId0_1_, person0_.name as name0_1_, person0_.age as age0_1_, idcard1_.id as id1_0_, idcard1_.userful_life as userful2_1_0_, idcard1_.person_Id as person3_1_0_ from addressbooksample.person person0_ left outer join addressbooksample.id_card
idcard1_ on person0_.personId=idcard1_.id where person0_.personId=?虽然发出了关联查询语句，但关联条件是person表的主键和idCard表的主键，显然这是不对的

正确的<one-to-one>

<one-to-one name="idCard" cascade="save-update" property-ref="person"/>

说明：假如不配置property-ref=”person”,那么从person对象查找关联的idCard默认是通过主键查找的，配置了property-ref属性之后，查找关联的idCard,在对象模型上是根据关联对象的某个属性，而在关系模型上是根据表的非主键。