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。