Hibernate系列:映射一对一连接表单向关联

SQL：C_E_JOIN_SINGLE为中间表

create table car_join_single (      
    id number(10,0) not null,      
    name varchar2(15) not null,      
    serial varchar2(30) not null,      
    manufacturer varchar2(50) not null,      
    producedate date,      
    primary key (id)      
);       
create table c_e_join_single(      
cid number(10,0) not null,      
eid number(10,0) not null unique,      
primary key (cid)      
);       
create table engine_join_single (      
    id number(10,0) not null,      
    model varchar2(20) not null,      
    manufacturer varchar2(50) not null,      
    producedate date,      
    primary key (id)      
);       
alter table C_E_JOIN_SINGLE      
  add constraint FK_CAR_JOIN_SINGLE foreign key (CID)      
  references CAR_JOIN_SINGLE (ID);      
alter table C_E_JOIN_SINGLE      
  add constraint FK_ENGINE_JOIN_SINGLE foreign key (EID)      
  references ENGINE_JOIN_SINGLE (ID);      



POJO：Car类有Engine类的一个引用(以下为省略写法)

public class Car implements Serializable      
{      
private Engine engine;      
}       
     
public class Engine implements Serializable      
{      
//没有car属性      
}     



XML:    Car.hbm.xml     Engine.hbm.xml(以下为省略写法)

<hibernate-mapping package="com.allanlxf.hibernate.association.o2o.entity">     
  <class name="Car" table="car_join">     
    <id name="id" column="id" type="integer">     
      <generator class="native" />     
    </id>     
    <join table="c_e_join_single">       
            <key column="cid"       
                 unique="true"/>       
            <many-to-one name="engine"       
                         column="eid"       
                         not-null="true"       
                         unique="true"     
                         fetch="join" />       
    </join>       
  </class>     
</hibernate-mapping>       
     
<hibernate-mapping package="com.allanlxf.hibernate.association.o2o.entity">     
  <class name="Engine" table="engine_join_single">     
    <id name="id" column="id" type="integer">     
      <generator class="native"/>     
    </id>     
  </class>     
</hibernate-mapping>      



a)  先保存其中一方得到那一方的id，然后再set并保存另一方
    session.save(engine);
    car.setEngine(engine);
    session.save(car);

b) session.get(Car.class, id),将得到一个外连接的sql，如下：
select
        car0_.id as id0_1_,
        car0_.name as name0_1_,
        car0_.serial as serial0_1_,
        car0_.manufacturer as manufact4_0_1_,
        car0_.producedate as produced5_0_1_,
        car0_1_.eid as eid1_1_,
        engine1_.id as id2_0_,
        engine1_.model as model2_0_,
        engine1_.manufacturer as manufact3_2_0_,
        engine1_.producedate as produced4_2_0_
    from
        car_join_single car0_
    inner join
        c_e_join_single car0_1_
            on car0_.id=car0_1_.cid
    inner join
        engine_join_single engine1_
            on car0_1_.eid=engine1_.id
    where
        car0_.id=?

one-to-one连接表单向关联映射总结: 需对在主控方的XML文件中设置join元素做连接表，表现为many-to-one。查询时可以使用fetch属性来提高查询效率。