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Hibernate Gossip: 一对多

在多对一中，User对Room是多对一的关系，User实例维护着对Room实例的参考，如果将这个关系反过来，由Room实例维护对多个User实例的数据，就是一对多的关系。

具体来说，可以设计User类别如下：

User.java

package onlyfun.caterpillar;

public class User {
    private Integer id;
    private String name;
    
    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;
    }
}

而在Room类别中，使用Set来记录多个User：

Room.java

package onlyfun.caterpillar; 

import java.util.Set;

public class Room { 
    private Integer id; 
    private String address;
    private Set users;
    
    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 Set getUsers() {
        return users;
    }

    public void setUsers(Set users) {
        this.users = users;
    } 
    
    public void addUser(User user) {
        users.add(user);        
    }
    
    public void removeUser(User user) {
        users.remove(user);
    }
}

这种方式即所谓单向一对多关系，也就是Room实例知道User实例的存在，而User实例则没有意识到Room实例。

（在多对一中，则是单向多对一关系，即User知道Room的存在，但Room不知道User的存在。）

在映射文件上，先来看看User.hbm.xml：

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"/> 
        
    </class> 

</hibernate-mapping>

在单向关系中，被参考的对象其映像文件就如单一实体一样的配置，接下来看看Room.hbm.xml，使用<one-to-many>卷标配置一对多：

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"/> 
                  
        <set name="users" table="user" cascade="all">
            <key column="room_id"/>
            <one-to-many class="onlyfun.caterpillar.User"/>
        </set>
    </class> 

</hibernate-mapping>

接着您可以如下储存对象：

User user1 = new User();
user1.setName("bush"); 
 
User user2 = new User(); 
user2.setName("caterpillar"); 
 
User user3 = new User(); 
user3.setName("momor"); 

Room room1 = new Room(); 
room1.setUsers(new HashSet());
room1.setAddress("NTU-M8-419");
room1.addUser(user1);
room1.addUser(user2);
        
Room room2 = new Room();
room2.setUsers(new HashSet());
room2.setAddress("NTU-G3-302");
room2.addUser(user3);

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
 
session.save(room1);  // cascade 操作
session.save(room2); 
 
tx.commit();
session.close();

数据库中将储存以下的表格：

mysql> select * from user;
+----+--------------+-----------+
| id    | name       | room_id |
+----+-------------+------------+
| 1 | bush       |      1 |
| 2 | caterpillar |      1 |
| 3 | momor      |      2 |
+----+-------------+------------+
3 rows in set (0.01 sec)

mysql> select * from room;
+----+------------------+
| id    | address      |
+----+------------------+
| 1    | NTU-M8-419 |
| 2    | NTU-G3-302 |
+----+------------------+
2 rows in set (0.00 sec)