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Hibernate Gossip: 多对多

在数据库表格上要进行多对多对应，可以藉由一个中介表格来完成，也就是藉由多对一、一对多来完成多对多关联。

多对多由于使用了中介表格，在查询效率不彰，且在程序的对象模式上，多对多会使得对象与对象之间彼此依赖，并不是一个很好的设计方式，在设计上应避免使用多对多关系。

如果一定要使用多对多关系的话，在表格上先如下建立：

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

CREATE TABLE user_server (
    user_id INT(11),
    server_id INT(11)
);

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

先设计User类别如下：

User.java

package onlyfun.caterpillar;

import java.util.Set;

public class User {
    private Integer id;
    private String name;
    private Set servers;
    
    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 Set getServers() {
        return servers;
    }

    public void setServers(Set servers) {
        this.servers = servers;
    }
}

再来设计Server类别如下：

Server.java

package onlyfun.caterpillar;

import java.util.Set;

public class Server {
    private Integer id;
    private String address;
    private Set users;
    
    public String getAddress() {
        return address;
    }
    
    public void setAddress(String address) {
        this.address = address;
    }
    
    public Integer getId() {
        return id;
    }
    
    public void setId(Integer id) {
        this.id = id;
    }
    
    public Set getUsers() {
        return users;
    }
    
    public void setUsers(Set users) {
        this.users = users;
    }
}

在映射文件上，使用<many-to-many>标签来完成映像关系：

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"/> 

        <set name="servers" 
             table="user_server" 
             cascade="save-update"> 
             
            <key column="user_id"/> 
            <many-to-many class="onlyfun.caterpillar.Server" 
                          column="server_id"/>            
        </set> 
    </class> 

</hibernate-mapping>

注意到cascade是设定为save-update，因为在多对多的关系中，很少因为删除其中之一，而所关联的实体都要一并删除的，所以设定save-update，表示在save或update时，一并对关联的对象进行对应的save或update。

Server.hbm.xml的定义如下：

Server.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.Server" table="server"> 

        <id name="id" column="id"> 
            <generator class="native"/> 
        </id> 

        <property name="address" type="java.lang.String"/> 
        
        <set name="users" 
             table="user_server" 
             inverse="true" 
             cascade="save-update"> 
              
             <key column="server_id"/> 
             <many-to-many class="onlyfun.caterpillar.User" 
                           column="user_id"/> 
        </set> 
    </class> 

</hibernate-mapping>

一个储存时的例子如下：

Server server1 = new Server();
server1.setAddress("PC-219"); 
server1.setUsers(new HashSet());
        
Server server2 = new Server(); 
server2.setAddress("PC-220"); 
server2.setUsers(new HashSet());
        
Server server3 = new Server(); 
server3.setAddress("PC-221");
server3.setUsers(new HashSet());
        
User user1 = new User(); 
user1.setName("caterpillar");
user1.setServers(new HashSet());
        
User user2 = new User(); 
user2.setName("momor");
user2.setServers(new HashSet());

// 多对多，互相参考
user1.getServers().add(server1); 
user1.getServers().add(server2); 
user1.getServers().add(server3); 
server1.getUsers().add(user1); 
server2.getUsers().add(user1); 
server3.getUsers().add(user1); 
        
user2.getServers().add(server1); 
user2.getServers().add(server3); 
server1.getUsers().add(user2); 
server3.getUsers().add(user2); 
        
Session session = sessionFactory.openSession(); 
Transaction tx= session.beginTransaction(); 
session.save(user1); 
session.save(user2); 
        
tx.commit();
session.close();

执行后数据库的内容如下：

mysql> select * from user;
+----+--------------+
| id    | name          |
+----+--------------+
| 1 | caterpillar |
| 2 | momor       |
+----+--------------+
2 rows in set (0.00 sec)

mysql> select * from user_serv
+----------+-------------+
| user_id | server_id |
+----------+-------------+
|       1 |            1 |
|       1 |      2 |
|       1 |         3 |
|       2 |         1 |
|       2 |         2 |
+----------+-------------+
5 rows in set (0.00 sec)

mysql> select * from server;
+----+-----------+
| id | address |
+----+-----------+
| 1    | PC-219 |
| 2    | PC-221 |
| 3    | PC-220 |
+----+-----------+
3 rows in set (0.00 sec)