<many-to-one>标签

many to one ： hibernate的多对一关系映射
（可以想象为多个用户对应一个组）
主要用在表中有外键的时候。 设置了<many-to-one>标签，hibernate在建表的时候，就会把该标签代表的对象的主键当做外键加入主表。在查询主表的时候，hibernate也可以利用映射关系，查出外键表的相应信息。
以下是例子：

Group.java    代表从表的对象

package com.ahuzl.hibernate;

public class Group {
    private int id;
    private String name;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

Group.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <class name="com.ahuzl.hibernate.Group" table="t_group">
        <id name="id" column="group_id">
            <generator class="native"/>
        </id>
        <property name="name" column="group_name"/>
    </class> 
</hibernate-mapping>

从表要设主键

User.java     代表主表的对象

package com.ahuzl.hibernate;

public class User {
    private int id;
    private String name;
    private Group group;

    public Group getGroup() {
        return group;
    }
    public void setGroup(Group group) {
        this.group = group;
    }
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

User.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <class name="com.ahuzl.hibernate.User" table="t_user">
        <id name="id" column="user_id">
            <generator class="native"/>
        </id>
        <property name="name" column="user_name"/>
        <many-to-one name="group" column="group_id" cascade="all"/>
    </class> 
</hibernate-mapping>

cascade="all"这个属性很重要，没有他的话，保存对象时会出错（org.hibernate.TransientObjectException），有了以后他会自动先保存从表的数据，再保存主表的数据，这样就不会出错了
Many2OneTest.java

package com.ahuzl.hibernate;

import org.hibernate.Session;
import junit.framework.TestCase;

public class Many2OneTest extends TestCase { 
    public void testReadByGetMethod1(){
        Session session = null;
        try{
            session = HibernateUtils.getSession();
            session.beginTransaction();

            Group group = new Group();
            group.setName("ahuzl");
//上面说到cascade这个属性，如果不写这个属性，那么先保存从表数据也是一个办法
            //session.save(group); 

            User user1 = new User();
            user1.setName("本田");
            user1.setGroup(group);

            User user2 = new User();
            user2.setName("豊田");
            user2.setGroup(group);

            session.save(user1);
            session.save(user2);
            session.getTransaction().commit();
        }catch(Exception e){
            e.printStackTrace();
            session.getTransaction().rollback();
        }finally{
            if (session != null){
                if (session.isOpen()){
                    session.close();
                }
            }
        }
    }
}

最后是查询的例子

Many2OneTest.java

package com.ahuzl.hibernate;

import org.hibernate.Session;
import junit.framework.TestCase;

public class Many2OneTest extends TestCase { 
    public void testReadByGetMethod3(){
        Session session = null;
        try{
            session = HibernateUtils.getSession();
            session.beginTransaction();
            User user = (User)session.load(User.class, 3);
            System.out.println("user.username : " + user.getName());
            System.out.println("user.usergroup.id : " + user.getGroup().getId());
            System.out.println("user.usergroup.name : " + user.getGroup().getName());
            session.getTransaction().commit();
        }catch(Exception e){
            e.printStackTrace();
            session.getTransaction().rollback();
        }finally{
            if (session != null){
                if (session.isOpen()){
                    session.close();
                }
            }
        }
    }
}

这样一来，不用自己做关联查询，hibernate就会帮你把值查出来，不过可以看到hibernate是用了两句SQL文，这个效率可能会是个问题。hibernate也提供了解决这个问题的办法，就是fetch="join"属性，配置了这个属性，hibernate就会发出外连接的SQL文

<many-to-one name="group" column="group_id" cascade="all" fetch="join"/&

最后要改的地方就是两个实体类的映射文件要纳入hibernate的配置文件中去
hibernate.cfg.xml

<!DOCTYPE hibernate-configuration PUBLIC
    "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
    <session-factory>
        <property name="hibernate.connection.url">jdbc:mysql://localhost/hibernate_many2one</property>
        <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="hibernate.connection.username">root</property>
        <property name="hibernate.connection.password">nimapiya</property>
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
        <property name="hibernate.show_sql">true</property>
        <property name="hibernate.hbm2ddl.auto">update</property>
        <mapping resource="com/ahuzl/hibernate/User.hbm.xml"/>
        <mapping resource="com/ahuzl/hibernate/Group.hbm.xml"/>
     </session-factory>
</hibernate-configuration&

评论

1 楼 kira616 2011-11-11  

<many-to-one