1.多对多

在关系映射的"多对多"案例基础上，使用基于注解实现一个学生对应多个教师，一个教师也可以对应多个学生的多对多关联关系。

项目结构图如下：

1.在com.dao包下修改StudentsMapper和新建TeachersMapper

//StudentsMapper.java
package com.dao;

import com.entity.IdCard;
import com.entity.Students;
import org.apache.ibatis.annotations.*;
import java.util.List;

public interface StudentsMapper {

    @Select("select * from students where sid=#{sid}")
    @Results(@Result(column = "cid",property = "card",javaType = IdCard.class, one = @One(select = "com.dao.IdCardMapper.getIdCardById")))
    public Students getStudentBySid(Integer sid);


    @Select("select s.*,c.code from students s,idcard c where s.cid = c.id and s.sid=#{sid}")
    @Results({
            @Result(property = "card.id",column = "cid" ),
            @Result(property = "card.code",column = "code")
    })
    public Students getStudentBySid2(Integer sid);

    //根据班级编号查询学生集合
    @Select("select * from students where class_room_id=#{classRoomId}")
    public List<Students> getStudentsByClassRoomId(Integer classRoomId);

    //根据学生编号查询带班级信息的学生资料
    @Select("select s.*,c.code,cr.cname from students s,idcard c,classroom cr where s.cid = c.id and s.class_room_id=cr.cid and s.sid=#{sid}")
    @Results({
            @Result(property = "card.id",column = "cid" ),
            @Result(property = "card.code",column = "code"),
            @Result(property = "classRoom.cid",column = "class_room_id"),
            @Result(property = "classRoom.cname",column = "cname")
    })
    public Students getStudentWithClassRoomBySid(Integer sid);

    //根据学生编号查询带教师集合信息的学生信息
    @Select("select * from students where sid = #{sid}")
    @Results({
            @Result(id=true,property = "sid",column = "sid"),
            @Result(property = "teachers",column = "sid",many=@Many(select = "com.dao.TeachersMapper.queryTeachersFromStudentsAndTeachersBySid"))
    })
    public Students getStudentWithTeachersBySid(Integer sid);

    //从中间表中根据教师编号查询学生集合
    @Select("select * from students where sid in (select sid from students_teachers where tid=#{tid})")
    public List<Students> getStudentsFromStudentsAndTeachersByTid(Integer tid);
}


//TeachersMapper.java

package com.dao;

import com.entity.Teachers;
import org.apache.ibatis.annotations.Many;
import org.apache.ibatis.annotations.Result;
import org.apache.ibatis.annotations.Results;
import org.apache.ibatis.annotations.Select;
import java.util.List;

public interface TeachersMapper {

    //从中间表中根据学生编号查询教师集合
    @Select("select * from teachers where tid in (select tid from students_teachers where sid=#{sid})")
    public List<Teachers> queryTeachersFromStudentsAndTeachersBySid(Integer sid);

    //根据教师编号查询带学生集合信息的教师信息
    @Select("select * from teachers where tid = #{tid}")
    @Results({
            @Result(id=true,property = "tid",column = "tid"),
            @Result(property = "stus",column = "tid",many=@Many(select = "com.dao.StudentsMapper.getStudentsFromStudentsAndTeachersByTid"))
    })
    public Teachers queryTeacherWithStudentsByTid(Integer tid);

}

2.在mybatis-config.xml添加映射接口。

    <!--映射文件-->
    <mappers>
        <mapper class="com.dao.StudentsMapper"/>
        <mapper class="com.dao.IdCardMapper"/>
        <mapper class="com.dao.ClassRoomMapper"/>
        <mapper class="com.dao.TeachersMapper"/>
    </mappers>

3.编写测试类

//StudentsMapperTest.java

    @Test
    public void testQueryStudentWithTeachersBySid(){
        SqlSession session = null;
        Students s = null;
        try {
            session = SqlSessionFactoryUtil.openSqlSession();
            StudentsMapper studentsMapper =  session.getMapper(StudentsMapper.class);
            s = studentsMapper.getStudentWithTeachersBySid(2);
            System.out.println(s);
        } catch (Exception ex) {
            ex.printStackTrace();
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }

//TeachersMapperTest.java

    @Test
    public void testQueryTeachersWithStudentsByTid(){
        SqlSession session = null;
        try {
            session = SqlSessionFactoryUtil.openSqlSession();
            TeachersMapper teachersMapper = session.getMapper(TeachersMapper.class);
            Teachers teacher= teachersMapper.queryTeacherWithStudentsByTid(1);
            System.out.println(teacher);
        } catch (Exception ex) {
            ex.printStackTrace();
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }