5.1 多对多关联关系

一个学生可以选多门课程，而一门课程可以由多个学生选。这就是典型的多对多关系。所谓多对多，其实是由两个互反的一对多关系组成。

一般情况下，多对多关系都会通过一个中间表来建立。course表，middle表，student表。

定义实体类：Course.java

package com.bjpowernode.beans;

public class Course {
    private Integer cid;
    private String cname;
    private Set<Student> students;
    public Integer getCid() {
        return cid;
    }
    ...
    getter and setter methods
    ...
}

定义实体类：Student.java

package com.bjpowernode.beans;

public class Student {
    private Integer sid;
    private String sname;
    private Set<Course> courses;
    public Integer getSid() {
        return sid;
    }
    ...
    getter and setter methods
    ...        
}

接口

package com.bjpowernode.dao;

public interface IStudentDao {
    Student selectStudentById(int sid);
}

映射文件

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.bjpowernode.dao.IStudentDao">

    <resultMap type="Student" id="studentlMapper">
        <id column="sid" property="sid"/>
        <result column="sname" property="sname"/>
        <collection property="courses" ofType="Course">
            <id column="cid" property="cid"/>
            <result column="cname" property="cname"/>
        </collection>
    </resultMap>

    <select id="selectStudentById" resultMap="studentlMapper">
        select sid, sname, cid, cname 
        from student, middle, course
        where sid=studentId and cid=courseId and sid=#{xxx}
    </select>

</mapper>

测试类

package com.bjpowernode.test;

public class MyTest {

    private IStudentDao dao;
    private SqlSession sqlSession;

    @Before
    public void Before() {
        sqlSession = MyBatisUtils.getSqlSession();
        //dao:是一个动态代理
        dao = sqlSession.getMapper(IStudentDao.class);
    }

    @After
    public void after() {
        if(sqlSession != null) {
            sqlSession.close();
        }
    }

    @Test
    public void test01() {
        Student student = dao.selectStudentById(1);
        System.out.println(student);
    }
}