Hibernate表关联-one-to-one

三种实现方式，已顾客与房间的关系为例：

0. DB代码：

    DROP TABLE IF EXISTS 'Customer'; 

    CREATE TABLE 'Customer' (

    'id int ( 11 ) NOT NULL auto_increment,

    'name' varchar(255 ) NOT NULL,

    PRIMARY KEY  (`id`)) ENGINE=InnoDB DEFAULT CHARSET=utf8;

 

    DROP TABLE IF EXISTS 'Room'; 

    CREATE TABLE 'Room' (

    'id' int ( 11 ) NOT NULL auto_increment,  

    'name' varchar(255 ) NOT NULL,  

    'customer_id' int ( 11 ) ,

    foreign key (' customer _id') references customer('id'), 

    PRIMARY KEY  ('id') ) ENGINE=InnoDB DEFAULT CHARSET=utf8; 

1. 实体代码：

    public   class Customer implements  java.io.Serializable { 

              private  Integer id;  

              private  String name;  

              private Room room;

              //省略构造函数，get/set方法

    }

    public   class Room implements  java.io.Serializable { 

              private  Integer id;  

              private  String name;  

              private Customer customer ;

              //省略构造函数，get/set方法

    }

2. 通过外键方式实现：

    Room.hbm.xml:

    <hibernate-mapping> 

             <class  name= "com.domain.Customer"  table= "Customer"  catalog= "Temp" >

                      < id name="id"  type= "java.lang.Integer" > 

                            <column name="id"  />

                            <generator class = "native"  />

                      </id>  

                      <property name="name"  type= "java.lang.String>

                                 <column name="name"  not- null = "true"  />

                      </property>

                      <!-- class 可以不写，因为根据name的值computer(属性)，会通过反射自动找到属于哪个类的 -->

                      <one-to-one cascade="all"  name= "customer"   class = "com.domain.Customer"  

                               property-ref= " customer " >

                      </one-to-one>

                </class >

     </hibernate-mapping> 

     Customer.hbm.xml:

     <hibernate-mapping>

           <class  name= "com.domain.Customer"  table= "Customer"  catalog= "Temp" >

                <id name="id"  type= "java.lang.Integer" >

                      <column name="id"  />

                      <generator class = "native"  />

                </id>

                <property name="name"  type= "java.lang.String" >

                      <column name="name"  not- null = "true"  />

                </property>

                <!-- many开头的是代表该表持有外键 -->

                <many-to-one  name="rom"   class = "com.domain.Room"   unique= "true">

                      <column name=" customer _id"  />

                </many-to-one> 

             </class >

      </hibernate-mapping> 

3. 通过主键方式，即一个表的主键有另一个表的主键决定，本例中Customer表的主键由Room表决定：

    Room.hbm.xml:

    <hibernate-mapping> 

             <class  name= "com.domain.Customer"  table= "Customer"  catalog= "Temp" >

                      < id name="id"  type= "java.lang.Integer" > 

                            <column name="id"  />

                            <generator class = "native"  />

                      </id>  

                      <property name="name"  type= "java.lang.String>

                                 <column name="name"  not- null = "true"  />

                      </property>

                      <!-- class 可以不写，因为根据name的值computer(属性)，会通过反射自动找到属于哪个类的 -->

                      <!--

                      <one-to-one cascade="all"  name= "customer"   class = "com.domain.Customer"  

                               property-ref= " customer " >

                      </one-to-one>

                      -->

                      <one-to-one name="customer" class = "com.domain.Customer" cascade="all" >

                </class >

     </hibernate-mapping> 

     Customer.hbm.xml:

     <hibernate-mapping>

           <class  name= "com.domain.Customer"  table= "Customer"  catalog= "Temp" >

                <id name="id"  type= "java.lang.Integer" >

                      <column name="id"  />

                      <!--

                      <generator class = "native" cascade="all"   />

                       -->

                      <!--Customer的主键由Room的主键决定，可以看成是外键-->      

                      <generator class = "foreign" >

                              <param name="property" >room</param>

                      </generator>

                      <!--或者使用下面的方式-->

                      <generator class="increment"></generator>

                </id>                

 

                <property name="name"  type= "java.lang.String" >

                      <column name="name"  not- null = "true"  />

                </property>

                <!-- many开头的是代表该表持有外键 -->

                <!--

                <many-to-one  name="rom"   class = "com.domain.Room"   unique= "true">

                      <column name=" customer _id"  />

                </many-to-one> 

                -->

                <one-to-one name="rom"   class = "com.domain.Room" />

                <!--或者使用下面的方式-->

                <one-to-one name="rom"   class = "com.domain.Room" constrained="true" />

             </class >

      </hibernate-mapping> 

4. 通过关系表实现：

    create table room_cust( 

              room_id int  not  null ,  

              cust_id int  not  null

    )ENGINE=InnoDB DEFAULT CHARSET=utf8; 

    配置文件：

    <!--optional= "true" 的意思是只有当room_id 和 cust_id 都不为空时才在关系表里插入。这样也会插入 2 遍，

       所以需要inverse= "true" 把一方设置为虚的。即不让插入到关系表中-->

    <join table=" room_cus t"  optional= "true"  inverse= "true" >

              <key column="cust_id" />

              <many-to-one name="room"  column= "room_id"  unique= "true" />

    </join>