一对多关联映射(one-to-many)
一对多关联映射利用了多对一关联映射原理
* 多对一关联映射:在多的一端加和一个外键指向一的一端,它维护的关系是多指向一的
* 一对多关联映射:在一的一端加和一个外键指向多的一端,它维护的关系是一指向多的
也就是说一对多和多对一的映射策略是一样的,只是站的角度不同,一般都作成双项的
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1 一对多关联映射(单项Classes--->Student)
在一一端维护关系的缺点:
* 如果将t_student表里的classesid这段设置为非空,则无法保存
* 因为不在student这一端维护关系,所以student不知道是哪个班的
所以需要发出多余的update语句来更新关系
<!--
Classes: Student:
private int id; private int id;
private String name; private String name;
private Set students;//必须用Set
-->
<class name="com.my.hibernate.Student">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
</class>
<class name="Classes">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
<set name="students">
<key column="classesid"/>
<one-to-many class="Student"/>
</set>
</class>
public void testSave1(){
Session session=null;
try{
session=HibernateUtils.getSession();
session.beginTransaction();
Student student1=new Student();
student1.setName("10");
session.save(student1);//先进行save操作
Student student2=new Student();
student2.setName("祖儿");
session.save(student2);//先进行save操作
Set students=new HashSet();
students.add(student1);
students.add(student2);
Classes classes=new Classes();
classes.setName("尚学堂");
classes.setStudents(students);
session.save(classes);
session.getTransaction().commit();
}catch(Exception e){
session.getTransaction().rollback();
e.printStackTrace();
}finally{
HibernateUtils.closeSession(session);
}
}
public void testLoad1(){
Session session=null;
try{
session=HibernateUtils.getSession();
session.beginTransaction();
Classes classes=(Classes)session.load(Classes.class, 3);
System.out.println(classes.getName());
Set students=classes.getStudents();
for(Iterator iter=students.iterator();iter.hasNext();){
Student student=(Student)iter.next();
System.out.print(student.getName()+";");
}
session.getTransaction().commit();
}catch(Exception e){
session.getTransaction().rollback();
e.printStackTrace();
}finally{
HibernateUtils.closeSession(session);
}
}
2 一对多关联映射(双项Classes<--->Student)
一对多双向关联映射:
* 在一一端的集合上用<key>,在对方表中加入一个外键指向一的一端
* 在多的一端采用<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,否则引用字段错误
如果在一的一端维护一对多关联关系,hibernate会发出多余的update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,所以我们可以从一的一端和多的一端维护关联关系,
如果设inverse为true,则我们只能从多的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是操作上的连锁反应
<!--
Classes: Student:
private int id; private int id;
private String name; private String name;
private Set students;//必须用 Set private Classes classes;
-->
<class name="Classes">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
<set name="students" inverse="true" cascade="all">
//cascade属性:none 不及连 save-update 插入或更新及连 delete删除及连 all
<key column="classesid"/>
<one-to-many class="Student"/>
</set>
</class>
<class name="com.my.hibernate.Student">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
<many-to-one name="classes" column="classesid" cascade="save-update"/>
</class>
public void testSave1(){
Session session=null;
try{
session=HibernateUtils.getSession();
session.beginTransaction();
//在多方维护
Classes classes=new Classes();
classes.setName("北青");
//session.save(classes);//加了cascade属性可以不用
Student student1=new Student();
student1.setName("黄不接10");
student1.setClasses(classes);
session.save(student1);
session.getTransaction().commit();
}catch(Exception e){
session.getTransaction().rollback();
e.printStackTrace();
}finally{
HibernateUtils.closeSession(session);
}
}
public void testLoad1(){
Session session=null;
try{
session=HibernateUtils.getSession();
session.beginTransaction();
//从多方读取
Student student=(Student)session.load(Student.class, 5);
System.out.println(student.getName());
System.out.println(student.getClasses().getName());
session.getTransaction().commit();
}catch(Exception e){
session.getTransaction().rollback();
e.printStackTrace();
}finally{
HibernateUtils.closeSession(session);
}
}
public void testSave2(){
Session session=null;
try{
session=HibernateUtils.getSession();
session.beginTransaction();
//在一方维护,自动转向多方
Classes classes=new Classes();
classes.setName("尚学堂");
Student student1=new Student();
student1.setName("10");
student1.setClasses(classes);
Student student2=new Student();
student2.setName("祖儿");
student2.setClasses(classes);
Set students=new HashSet();
students.add(student1);
students.add(student2);
classes.setStudents(students);
session.save(classes);
session.getTransaction().commit();
}catch(Exception e){
session.getTransaction().rollback();
e.printStackTrace();
}finally{
HibernateUtils.closeSession(session);
}
}
多对多关联映射(单项User---->Role)
<many-to-many/>标签
<set>标签中加入属性table="t_user_role"创建关联表
<!--
User: Role:
private int id; private int id;
private String name; private String name;
private Set roles;
-->
<class name="com.my.hibernate.Role">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
</class>
<class name="User">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
<set name="roles" table="t_user_role">
<key column="userid"/>
<many-to-many class="Role" column="roleid"/>
</set>
</class>
public void testSave1(){
Session session=null;
try{
session=HibernateUtils.getSession();
session.beginTransaction();
Role role1=new Role();
role1.setName("1111");
Role role2=new Role();
role2.setName("2222");
Role role3=new Role();
role3.setName("3333");
session.save(role1);
session.save(role2);
session.save(role3);
User user1=new User();
user1.setName("user1");
Set user1Role=new HashSet();
user1Role.add(role1);
user1Role.add(role2);
user1.setRoles(user1Role);
User user2=new User();
user2.setName("user2");
Set user2Role=new HashSet();
user2Role.add(role2);
user2Role.add(role3);
user2.setRoles(user2Role);
User user3=new User();
user3.setName("user3");
Set user3Role=new HashSet();
user3Role.add(role2);
user3Role.add(role3);
user3Role.add(role1);
user3.setRoles(user3Role);
session.save(user1);
session.save(user2);
session.save(user3);
session.getTransaction().commit();
}catch(Exception e){
session.getTransaction().rollback();
e.printStackTrace();
}finally{
HibernateUtils.closeSession(session);
}
}
public void testLoad1(){
Session session=null;
try{
session=HibernateUtils.getSession();
session.beginTransaction();
User u1=(User)session.load(User.class, 1);
System.out.println(u1.getName());
for(Iterator iter=u1.getRoles().iterator();iter.hasNext();){
System.out.println(((Role)iter.next()).getName());
}
session.getTransaction().commit();
}catch(Exception e){
session.getTransaction().rollback();
e.printStackTrace();
}finally{
HibernateUtils.closeSession(session);
}
}
多对多关联映射(双项User<---->Role)
<many-to-many/>标签
映射方法:
<set name="users" table="t_user_role" order-by="userid">
<key column="roleid"/>
<many-to-many class="com.my.hibernate.User" column="userid"/>
</set>
table属性值必须和单向关联的table属性值一致
<key>中column属性值要与单向关联中的<many-to-many>标签中的column属性值一致
<many-to-many>中column属性值要与单向关联中的<key>标签中的column属性值一致
<!--
User: Role:
private int id; private int id;
private String name; private String name;
private Set roles; private Set users;
-->
<class name="com.my.hibernate.Role">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
<set name="users" table="t_user_role" order-by="userid">
<key column="roleid"/>
<many-to-many class="com.my.hibernate.User" column="userid"/>
</set>
</class>
<class name="User">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
<set name="roles" table="t_user_role">
<key column="userid"/>
<many-to-many class="Role" column="roleid"/>
</set>
</class>
public void testLoad2(){
Session session=null;
try{
session=HibernateUtils.getSession();
session.beginTransaction();
Role role=(Role)session.load(Role.class, 3);
System.out.println(role.getName());
for(Iterator iter=role.getUsers().iterator();iter.hasNext();){
System.out.println(((User)iter.next()).getName());
}
session.getTransaction().commit();
}catch(Exception e){
session.getTransaction().rollback();
e.printStackTrace();
}finally{
HibernateUtils.closeSession(session);
}
}
set 不可重复
posted on 2009-11-03 16:04
junly 阅读(644)
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hibernate/orm