【hibernate框架】多对多单向关联(XML实现)

什么情况下是多对多？
举个例子：一个老师可以教多个学生，每个学生也可以被多个老师教。这就是多对多的情况。
在数据库表中这种关系该怎么设计？很简单，加一张中间表。


设计模型：
table_student id<int> <pk>
table_teacher id<int> <pk>


中间表：
T_S t_id<int> <pk,fk2> s_id<int> <pk,fk1>


比如老师2教学生1，在中间表中就存2,1
老师2教学生3，在中间表中就存2,3
这就可以表示多对多的关系了。


hibernate中XML配置文件的实现：
Student.java:

package cn.edu.hpu.many2many;


public class Student {
  private int id;
  private String name;  


  public int getId() {
    return id;
  }
  public void setId(int id) {
    this.id = id;
  }
  public String getName() {
    return name;
  }
  public void setName(String name) {
    this.name = name;
  }
  
}

Teacher.java:

package cn.edu.hpu.many2many;


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


import cn.edu.hpu.many2many.Student;


public class Teacher {
  private int id;
  private String name;
  private Set<Student> students=new HashSet<Student>();
  //选择set的原因是因为，set互相之间不会有重复的
  //跟数据库模型比较匹配


  public int getId() {
    return id;
  }
  public void setId(int id) {
    this.id = id;
  }
  public String getName() {
    return name;
  }
  public void setName(String name) {
    this.name = name;
  }
  
  public Set<Student> getStudents() {
    return students;
  }
  public void setStudents(Set<Student> students) {
    this.students = students;
  }
  
}

配置文件：

Teacher.hbm.xml:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">


<hibernate-mapping package="cn.edu.hpu.many2many">
    <class name="Teacher" table="xm_teacher" dynamic-update="true">
        <!-- 主键 这里的主键名字刚好也是'id'-->
            <id name="id" column="id">
                <generator class="native"></generator>
            </id>  
        
        <property name="name"></property> 
        <set name="students" table="t_s">
           <key column="teacher_id"></key><!-- 指定关联字段的名字叫什么 -->
           <many-to-many class="cn.edu.hpu.many2many.Student" column="student_id"/>
        </set>
    </class>
</hibernate-mapping>

Student.hbm.xml:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">


<hibernate-mapping package="cn.edu.hpu.many2many">
    <class name="Student" table="xm_student" dynamic-update="true">
        <!-- 主键 这里的主键名字刚好也是'id'-->
            <id name="id" column="id">
                <generator class="native"></generator>
            </id>  
        
        <property name="name"></property> 
        
    </class>
</hibernate-mapping>

在hibernate.cfg.xml中配置:

<mapping resource="cn/edu/hpu/many2many/Student.hbm.xml"/> 

 <mapping resource="cn/edu/hpu/many2many/Teacher.hbm.xml"/>

生成的建表语句：

create table t_s (

         teacher_id integer not null,

         student_id integer not null,

         primary key (teacher_id, student_id)

     )



     create table xm_student (

         id integer not null auto_increment,

         name varchar(255),

         primary key (id)

     )



     create table xm_teacher (

         id integer not null auto_increment,

         name varchar(255),

         primary key (id)

     )



     alter table t_s 

         add index FK1BF687B5893C2 (teacher_id), 

         add constraint FK1BF687B5893C2 

         foreign key (teacher_id) 

         references xm_teacher (id)



     alter table t_s 

         add index FK1BF686ABD4922 (student_id), 

         add constraint FK1BF686ABD4922 

         foreign key (student_id) 

         references xm_student (id)

除了生成了xm_student和xm_teacher两张表之外，还生成了一张中间表t_s