10hibernate_one2many_1

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

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：在多的一端加入一个外键指向一的一端，它维护的关系是多指向一
一对多关联映射：在多的一端加入一个外键指向一的一端，它维护的关系是一指向多

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

在一一端维护关系的缺点：
    * 如果将t_student表里的classesid字段设置为非空，则无法保存
    * 因为不是在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


 一方：Classes.java

package com.bjsxt.hibernate;

import java.util.Set;

public class Classes {
    
    private int id;
    
    private String name;
    
    private Set students; 
    
    <hibernate-mapping package="com.bjsxt.hibernate">
    <class name="Classes" table="t_classes">
        <id name="id">
            <generator class="native"/>
        </id>
        <property name="name"/>
        <set name="students">
            <key column="classesid"/>
            <one-to-many class="Student"/>
        </set>
    </class>
</hibernate-mapping>

-------------------------------------------------------------------------------
       多方：
    public class Student {
    
    private int id;
    
    private String name;
    
    <hibernate-mapping>
    <class name="com.bjsxt.hibernate.Student" table="t_student">
        <id name="id">
            <generator class="native"/>
        </id>
        <property name="name"/>
    </class>
</hibernate-mapping>
-------------------------------------------------------------------------------
ExportDB:

create table t_classes (id integer not null auto_increment, name varchar(255), primary key (id))

create table t_student (id integer not null auto_increment, name varchar(255), classesid integer, primary key (id))

alter table t_student add index FK4B9075708EBC77F7 (classesid), add constraint FK4B9075708EBC77F7 foreign key (classesid) references t_classes (id)     

public void testSave1() {
        Session session = null;
        try {
            session = HibernateUtils.getSession();
            session.beginTransaction();
            
            Student student1 = new Student();
            student1.setName("10");
            session.save(student1);
            
            Student student2 = new Student();
            student2.setName("祖儿");
            session.save(student2);
            
            Set students = new HashSet();
            students.add(student1);
            students.add(student2);
            
            Classes classes = new Classes();
            classes.setName("尚学堂");
            classes.setStudents(students);
            
            //可以正确保存
            session.save(classes);
            
            session.getTransaction().commit();
        }catch(Exception e) {
            e.printStackTrace();
            session.getTransaction().rollback();
        }finally {
            HibernateUtils.closeSession(session);
        }
    }    
测试结果：
Hibernate: insert into t_student (name) values (?)
Hibernate: insert into t_student (name) values (?)
Hibernate: insert into t_classes (name) values (?)
Hibernate: update t_student set classesid=? where id=?
Hibernate: update t_student set classesid=? where id=?    
-------------------------------------------------------------------------------
public void testLoad1() {
        Session session = null;
        try {
            session = HibernateUtils.getSession();
            session.beginTransaction();
            
            Classes classes = (Classes)session.load(Classes.class, 1);
            System.out.println("classes.name=" + classes.getName());
            Set students = classes.getStudents();
            for (Iterator iter=students.iterator(); iter.hasNext();) {
                Student student = (Student)iter.next();
                System.out.println("student.name=" + student.getName());
            }
            session.getTransaction().commit();
        }catch(Exception e) {
            e.printStackTrace();
            session.getTransaction().rollback();
        }finally {
            HibernateUtils.closeSession(session);
        }
    }        
}

测试结果：
Hibernate: select classes0_.id as id0_0_, classes0_.name as name0_0_ from t_classes classes0_ where classes0_.id=?
classes.name=尚学堂
Hibernate: select students0_.classesid as classesid1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_ from t_student students0_ where students0_.classesid=?
student.name=祖儿
student.name=10


mysql> desc t_classes;
+-------+--------------+------+-----+---------+----------------+
| Field | Type         | Null | Key | Default | Extra          |
+-------+--------------+------+-----+---------+----------------+
| id    | int(11)      | NO   | PRI | NULL    | auto_increment |
| name  | varchar(255) | YES  |     | NULL    |                |
+-------+--------------+------+-----+---------+----------------+
2 rows in set (0.08 sec)

mysql> desc t_student;
+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| id        | int(11)      | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | YES  |     | NULL    |                |
| classesid | int(11)      | YES  | MUL | NULL    |                |
+-----------+--------------+------+-----+---------+----------------+
3 rows in set (0.02 sec)