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,
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

转载请注明出处:http://blog.csdn.net/acmman