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Hibernate Gossip: 一对一（唯一外键关联）

现在考虑每一个User配给一间Room，形成一对一，user表格透过room_id作为外键参考至room：

在表格建立方面，使用多对一中的表格建立语句就可以了：

CREATE TABLE user (
    id INT(11) NOT NULL auto_increment PRIMARY KEY,
    name VARCHAR(100) NOT NULL default '',
    room_id INT(11)
);

CREATE TABLE room (
    id INT(11) NOT NULL auto_increment PRIMARY KEY,
    address VARCHAR(100) NOT NULL default ''
);

对象方面，User的实例会参考至Room实例，而Room实例也参考至User实例：

User.java

package onlyfun.caterpillar;

public class User {
    private Integer id;
    private String name;
    private Room room;
    
    public User() {
    }

    public Integer getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public Room getRoom() {
        return room;
    }

    public void setRoom(Room room) {
        this.room = room;
    }
}

Room.java

package onlyfun.caterpillar; 

public class Room { 
    private Integer id; 
    private String address;
    private User user;
    
    public Room() {
    }
    
    public Integer getId() { 
        return id; 
    } 
    public void setId(Integer id) { 
        this.id = id; 
    }    
    
    public String getAddress() { 
        return address; 
    }
    
    public void setAddress(String address) { 
        this.address = address; 
    }

    public User getUser() {
        return user;
    }

    public void setUser(User user) {
        this.user = user;
    }
}

使用外键来完成一对一，其实就是限制多对一关系中，「多」的一方只能有一个参考至「一」的一方，也就是多对一关系的一个特例，这可以在映射文件中使用<many-to-one>卷标时，加上"unique"属性来设定，例如：

User.hbm.xml

<?xml version="1.0" encoding="utf-8"?> 
<!DOCTYPE hibernate-mapping 
    PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 

<hibernate-mapping> 

    <class name="onlyfun.caterpillar.User" table="user"> 
        <id name="id" column="id" type="java.lang.Integer"> 
            <generator class="native"/> 
        </id> 

        <property name="name" column="name" type="java.lang.String"/> 
        
        <many-to-one name="room" 
                     column="room_id" 
                     class="onlyfun.caterpillar.Room"
                     cascade="all"
                     outer-join="true"
                     unique="true"/>    
    </class> 

</hibernate-mapping>

到这边为止，单向一对一的映像已经完成，如果要再完成双向一对一的关系，则可以在Room.hbm.xml中使用<one-to-one>标签来定义：

Room.hbm.xml

<?xml version="1.0" encoding="utf-8"?> 
<!DOCTYPE hibernate-mapping 
    PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 

<hibernate-mapping> 

    <class name="onlyfun.caterpillar.Room" table="room"> 
        <id name="id" column="id"> 
            <generator class="native"/> 
        </id> 

        <property name="address" 
                  column="address" 
                  type="java.lang.String"/> 
                  
        <one-to-one name="user"
                    class="onlyfun.caterpillar.User"
                    property-ref="room"/>
    </class> 

</hibernate-mapping>

在<one-to-one>中，property-ref告诉Hibernate，查询出user并将其参考至room。

一个储存的例子如下：

User user1 = new User();
user1.setName("bush"); 
Room room1 = new Room(); 
room1.setAddress("NTU-M8-419");
user1.setRoom(room1);
        
User user2 = new User(); 
user2.setName("caterpillar"); 
Room room2 = new Room(); 
room2.setAddress("NTU-M8-418");
user2.setRoom(room2);
        
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
 
session.save(user1);
session.save(user2);
        
tx.commit();
session.close();

在查询Room时，User也会一加载，例如：

Session session = sessionFactory.openSession();
Room room = (Room) session.load(Room.class, new Integer(23));
System.out.println(room.getUser().getName());
session.close();

上面的查询程序，Hibernate将使用以下的SQL：

Hibernate:
select room0_.id as id1_, room0_.address as address1_1_, user1_.id as
id0_, user1_.name as name0_0_, user1_.room_id as room3_0_0_ from room
room0_ left outer join user user1_ on room0_.id=user1_.room_id where
room0_.id=?