关于Hibernate多对多拆分为多个一对多的问题.

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zhanghaidang

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发表文章: 5
注册时间: 2007年04月16日 14:36

关于Hibernate多对多拆分为多个一对多的问题.	2007年05月14日 21:57
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关于Hibernate多对多拆分为多个一对多的问题.
开发环境为:MyEclipse5.1
数据库:
create table student
(
sid int(8) not null auto_increment,
sname varchar(30) not null default '',
primary key(sid)
)engine=innodb default charset=gbk;

create table teacher
(
tid int(8) not null auto_increment,
tname varchar(30) not null default '',
primary key(tid)
)engine=innodb default charset=gbk;

create table student_teacher_link
(
id int(8) not null auto_increment,
sid int(8),
tid int(8),
primary key(id),
foreign key(sid) references student(sid),
foreign key(tid) references teacher(tid)
)engine=innodb default charset=gbk;

然后通过MyEclipse自动生成功能,生成以下hbm.xml和POJO类文件
#学生类
public class Student{
private Integer sid;
private String sname;
private Set studentTeacherLinks = new HashSet(0);

public Student() {}

public Student(String sname) {
this.sname = sname;
}

public Student(String sname, Set studentTeacherLinks) {
this.sname = sname;
this.studentTeacherLinks = studentTeacherLinks;
}

/*一系列set/get方法*/
}
#老师类
public class Teacher{
private Integer tid;
private String tname;
private Set studentTeacherLinks = new HashSet(0);
public Teacher() {}
public Teacher(String tname) {
this.tname = tname;
}

public Teacher(String tname, Set studentTeacherLinks) {
this.tname = tname;
this.studentTeacherLinks = studentTeacherLinks;
}
/*一系列set/get方法*/
}
#主键关联类
public class StudentTeacherLink{
private Integer id;
private Teacher teacher;
private Student student;

public StudentTeacherLink() {}
public StudentTeacherLink(Teacher teacher, Student student) {
this.teacher = teacher;
this.student = student;
}
/*一系列set/get方法*/
}

zhanghaidang

悄悄话
发表文章: 5
注册时间: 2007年04月16日 14:36

re:关于Hibernate多对多拆分为多个一对多的问题.

2007年05月14日 21:57

三个映射文件
#Studnet.hbm.xml
<class name="com.hibernate.pojo.Student" table="student">
<id name="sid" type="java.lang.Integer">
<column name="sid" />
<generator class="native" />
</id>
<property name="sname" type="java.lang.String">
<column name="sname" length="30" not-null="true" />
</property>
<set name="studentTeacherLinks" inverse="true" lazy="true">
<key>
<column name="sid" unique="true" />
</key>
<one-to-many class="com.hibernate.pojo.StudentTeacherLink" />
</set>
</class>

#Teacher.hbm.xml
<class name="com.hibernate.pojo.Teacher" table="teacher">
<id name="tid" type="java.lang.Integer">
<column name="tid" />
<generator class="native" />
</id>
<property name="tname" type="java.lang.String">
<column name="tname" length="30" not-null="true" />
</property>
<set name="studentTeacherLinks" inverse="true" lazy="true">
<key>
<column name="tid" unique="true" />
</key>
<one-to-many class="com.hibernate.pojo.StudentTeacherLink" />
</set>
</class>

#StudentTeacherLink.hbm.xml
<class name="com.hibernate.pojo.StudentTeacherLink" table="student_teacher_link">
<id name="id" type="java.lang.Integer">
<column name="id" />
<generator class="native" />
</id>
<many-to-one name="teacher" class="com.hibernate.pojo.Teacher" cascade="save-update">
<column name="tid" unique="true" />
</many-to-one>
<many-to-one name="student" class="com.hibernate.pojo.Student" cascade="save-update">
<column name="sid" unique="true" />
</many-to-one>
</class>

测试类
public class UtilDAO {
public static void saveStudent(){
//两个学生,两个老师
Student stu1 = new Student("亚当");
Student stu2 = new Student("夏娃");
Teacher tea1 = new Teacher("张老师");
Teacher tea2 = new Teacher("李老师");
//相互关联
StudentTeacherLink st1 = new StudentTeacherLink(tea1,stu1);
StudentTeacherLink st2 = new StudentTeacherLink(tea1,stu2);
StudentTeacherLink st3 = new StudentTeacherLink(tea2,stu1);
StudentTeacherLink st4 = new StudentTeacherLink(tea2,stu2);

Session session = null;
Transaction tr = null;
try{
session = HibernateSessionFactory.getSession();
tr = session.beginTransaction();
//保存数据
session.saveOrUpdate(st1);
session.saveOrUpdate(st2);
session.saveOrUpdate(st3);
session.saveOrUpdate(st4);
tr.commit();
}catch(Exception e){
e.printStackTrace();
tr.rollback();
}finally{
session.close();
}
}

public static void printStudent(){
Session session = null;
Transaction tr = null;

try{
session = HibernateSessionFactory.getSession();
tr = session.beginTransaction();
//查找老师ID为1的老师
Query query = session.createQuery("from Teacher t where t.tid=1");
//取得老师对象
Teacher ter = (Teacher)query.uniqueResult();
//???这里我不知道该不该这样获取学生集合
Set set = ter.getStudentTeacherLinks();
Iterator it = set.iterator();
while(it.hasNext()){
//***在这里取出学生对象时,发生异常了(java.lang.ClassCastException: com.hibernate.pojo.StudentTeacherLink)
Student stu = (Student)it.next();
System.out.println(stu.getSname());
}
tr.commit();
}catch(Exception e){
e.printStackTrace();
tr.rollback();
}finally{
session.close();
}
}
public static void main(String[] args){
//UtilDAO.saveStudent();
UtilDAO.printStudent();
}
}

问题:
1.请问:我能这样获取学生集合吗?
Set set = ter.getStudentTeacherLinks();

2.请问:我这样关联学生和老师的对象对吗?
StudentTeacherLink st1 = new StudentTeacherLink(tea1,stu1);

3.我如果想通过老师对象来查看对应的学生集合该怎么做呢?

banq

悄悄话
发表文章: 9528
注册时间: 2002年08月03日 17:08

回复:关于Hibernate多对多拆分为多个一对多的问题.

2007年05月16日 11:02

你使用Hibernate的方式就是错误的。
你是先有数据库:>create table student

所以产生后面琐碎问题，就象使用扳手工具，你是倒过来拿，你还要求扳手帮你做更多活，怎么可能。

相关贴：
http://www.jdon.com/jivejdon/thread/31684.html

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