hibernate 一对多one-to-many 单向 and 双向（many-to-one),inverse(反转）
hibernate 一对多 单向classes---->student (one--->many)
类：
one
public class Classes {
private Integer id;
private String name;
private Set students;
}
many
public class Student {//不用动
private Integer id;
private String name;
}
hbm.xml
Classes.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 package="com.model">
<class name="Classes" table="classes" >
<id name="id" column="id" type="java.lang.Integer">
<generator class="native" />
</id>
<property name="name" column="name" type="java.lang.String" />
<set name="students">
<key column="class_id" />
<one-to-many class="com.model.Student" />
</set>
</class>
< /hibernate-mapping>
Student.hbm.xml(不用动）
<?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.zd.model">
<class name="Student" table="student" >
<id name="id" column="id" type="java.lang.Integer">
<generator class="native" />
</id>
<property name="name" column="name" length="50" type="java.lang.String" />
</class>
< /hibernate-mapping>
测试用例：
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
import junit.framework.TestCase;
import org.hibernate.Session;
import org.hibernate.Transaction;
import com.model.Classes;
import com.model.Student;
public class OneToManyTest extends TestCase {
public void testSave1(){
Session session = null;
Transaction ta = null;
try{
session = HibernateUtil.getSession();
ta = session.beginTransaction();
Student stu1 = new Student();
stu1.setName("z3");
Student stu2 = new Student();
stu2.setName("l4");
session.save(stu1); //一定要先保存,要不是瞬时对象
session.save(stu2);
Set stuSet = new HashSet();
stuSet.add(stu1);
stuSet.add(stu2);
Classes c = new Classes();
c.setName("Java Class");
c.setStudents(stuSet);
session.save(c);
ta.commit();
}catch(Exception e){
e.printStackTrace();
if(ta != null){
ta.rollback();
}
}finally{
//关闭session， user变为detached离线对象
HibernateUtil.closeSession(session);
}
}
Hibernate: insert into student (name) values (?)
Hibernate: insert into student (name) values (?)
Hibernate: insert into classes (name) values (?)
Hibernate: update student set class_id=? where id=?
Hibernate: update student set class_id=? where id=?
注：表student 的字段class_id必须null（若为not null，就会报错）。多了2条update， 所以one-to-many保存时，不太好，不推荐。
public void testGet1(){
Session session = null;
Transaction ta = null;
try{
session = HibernateUtil.getSession();
ta = session.beginTransaction();
Classes c = (Classes) session.get(Classes.class, new Integer(2));
System.out.println("Class.name=" + c.getName());
Set stuSet = c.getStudents();
if(stuSet != null && !stuSet.isEmpty()){
for(Iterator it = stuSet.iterator(); it.hasNext();){
Student s = (Student) it.next();
System.out.println("student.name=" + s.getName());
}
}
ta.commit();
}catch(Exception e){
e.printStackTrace();
if(ta != null){
ta.rollback();
}
}finally{
//关闭session， user变为detached离线对象
HibernateUtil.closeSession(session);
}
Hibernate: select classes0_.id as id0_0_, classes0_.name as name0_0_ from classes classes0_ where classes0_.id=?
Class.name=Java Class
Hibernate: select students0_.class_id as class3_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_ from student students0_ where students0_.class_id=?
student.name=l4
student.name=z3
}
小结：one-to-may 是只配在one这端，但many另一端的表中要用外键与这相对应
<set name="students"> //name : one 中的set变量
<key column="class_id" /> //column 表student的外键字段class_id
<one-to-many class="com.model.Student" /> //class指many的类
</set>
===============================================================
双向（many-to-one) ，在多的一端配个多对一：
修改如下：
类：
public class Student {
private Integer id;
private String name;
private Classes classes;
｝
hbm.xml
Student.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 package="com.zd.model">
<class name="Student" table="student" >
<id name="id" column="id" type="java.lang.Integer">
<generator class="native" />
</id>
<property name="name" column="name" length="50" type="java.lang.String" />
<many-to-one name="classes" column="class_id"></many-to-one>
</class>
< /hibernate-mapping>
测试用例：
public void testSave2(){
Session session = null;
Transaction ta = null;
try{
session = HibernateUtil.getSession();
ta = session.beginTransaction();
Classes c = new Classes();
c.setName("php");
Student stu1 = new Student();
stu1.setName("a1");
stu1.setClasses(c);
Student stu2 = new Student();
stu2.setName("a2");
stu2.setClasses(c);
session.save(c); //先保存班级
session.save(stu1);
session.save(stu2);
ta.commit();
}catch(Exception e){
e.printStackTrace();
if(ta != null){
ta.rollback();
}
}finally{
//关闭session， user变为detached离线对象
HibernateUtil.closeSession(session);
}
}
================================================
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
<set name="students" inverse="true"> //inverse为true，则我们只能从多一端来维护关联关系
<key column="class_id" />
<one-to-many class="com.zd.model.Student" />
</set>
inverse和cascade
* inverse是关联关系的控制方向
* cascade操作上的连锁反应
<div class="tit">Hibernate之中inverse与cascade的异同
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