8.Hibernate的一对多和多对一双向关联映射

核心代码：

<many-to-one name="classes" column="classesId"/>

<set name="students" inverse="true" cascade="all">
      <key column="classesId" />
      <one-to-many class="Students" />
 </set>

 

hihernate一对多关联映射（双向Classes<----->Student）

一对多双向关联映射：
    * 在一一端的集合上使用<key>，在对方表中加入一个外键指向一一端
    * 在多一端采用<many-to-one>
   
注意：<key>标签指定的外键字段必须和<many-to-one>指定的外键字段一致，否则引用字段的错误
   
如果在”一“一端维护一对多关联关系，hibernate会发出多余的udpate语句，所以我们一般在多
的一端来维护关联关系

关于inverse属性：
    inverse主要用在一对多和多对多双向关联上，inverse可以被设置到集合标签<set>上，
    默认inverse为false，所以我们可以从”一“一端和”多“一端维护关联关系，
    如果设置成inverse为true，则我们只能从多一端来维护关联关系
   
    注意：inverse属性，只影响数据的存储，也就是持久化
       
inverse和cascade
    * inverse是关联关系的控制方向
    * cascade操作上的连锁反应

 

一：实体类：

package com.tao.hibernate.one2many2;

import java.util.Set;

public class Classes {
	private Integer id;
	private String name;
	private Set<Students> students;

	public Classes() {
		super();
	}

	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<Students> getStudents() {
		return students;
	}

	public void setStudents(Set<Students> students) {
		this.students = students;
	}

}

 

package com.tao.hibernate.one2many2;

public class Students {
	private Integer id;
	private String name;
	private Classes classes;

	public Students() {
		super();
	}

	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 Classes getClasses() {
		return classes;
	}

	public void setClasses(Classes classes) {
		this.classes = classes;
	}

}

 

二：映射文件：

<?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 package="com.tao.hibernate.one2many2" auto-import="false">
	<class name="Classes" table="t_classes">
		<id name="id" column="id" type="integer">
			<generator class="native" />
		</id>
		<property name="name" column="name" />
		<set name="students" inverse="true" cascade="all">
			<key column="classesId" />
			<one-to-many class="Students" />
		</set>
	</class>
</hibernate-mapping>

 

<?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 package="com.tao.hibernate.one2many2" auto-import="false">
	<class name="Students" table="t_students">
		<id name="id" column="id" type="integer">
			<generator class="native" />
		</id>
		<property name="name" column="name" />
		<many-to-one name="classes" column="classesId"/>
	</class>
</hibernate-mapping>

 

三：测试类：

package com.tao.hibernate.one2many2;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

import org.hibernate.Session;
import org.hibernate.Transaction;
import org.junit.Test;

import com.tao.hibernate.HibernateUtils;

public class TestMain {
	Session session = HibernateUtils.getFactory().openSession();

	@Test
	public void testSave() {
		Students s1 = new Students();
		s1.setName("tao");

		Students s2 = new Students();
		s2.setName("war");

		Classes c = new Classes();
		c.setName("148班");
		Set<Students> students = new HashSet<Students>();
		students.add(s1);
		students.add(s2);
		c.setStudents(students);

		s1.setClasses(c);
		s2.setClasses(c);

		Transaction ts = session.beginTransaction();
		ts.begin();
		session.save(c);
		ts.commit();
	}

	@Test
	public void testLoad() {
		Session session = HibernateUtils.getFactory().openSession();
		Classes c = (Classes) session.load(Classes.class, 1);
		for (Iterator<Students> iter = c.getStudents().iterator(); iter
				.hasNext();) {
			System.out.println(iter.next().getName());
		}
	}

	@Test
	public void testLoad2() {
		Session session = HibernateUtils.getFactory().openSession();
		Students s = (Students) session.load(Students.class, 1);
		System.out.println(s.getName());
		System.out.println(s.getClasses().getName());
	}

}