Mybatis一对多的处理

表的设计和Mybatis多对一相同

表的设计 

利用Navicat for mysql建表，然后在查看菜单点击ER图表（概念模型） 如下： 

设计好的表ER图表 

上述表 student tid设置外键参考teacher的id，所以建好表后两者都为空要先添加teacher的数据。 

多个学生对于一个老师 

实体类 

Teacher

public class Teacher {
    private int id;
    private String name;
    private List<Student>  students;
    //省略set/get方法
}
           

Student

public class Student {
    private int id;
    private String name;
    //省略set/get方法
}
           

编写teacher.mapper.xml映射文件

两种处理方式 

第一种 结果嵌套

<mapper namespace="cn.sxt.entity.teacher.mapper">
    <select id="getTeacher" resultMap="TeacherStudent">
        select  s.id sid,s.name sname,s.tid stid ,t.id tid,t.name tname from student s,teacher t where s.tid=t.id and tid=#{id}
    </select>
    <resultMap  type="Teacher"  id="TeacherStudent">
        <id column="tid"  property="id"></id>
        <result column="tname" property="name"></result>
        <collection  property="students"  javaType="ArrayList"   ofType="Student">
            <id column="sid"  property="id"></id>
            <result column="sname"  property="name"></result>
        </collection>
    </resultMap>
</mapper>
           

第二种 查询嵌套 

teacher.mapper.xml

<select id="getTeacher"  resultMap="TeacherStudent">
        select * from teacher where id=#{id}
    </select>
    <resultMap id="TeacherStudent"  type="Teacher">
        <collection  property="students"  ofType="Student"  column="id"  select="cn.sxt.entity.student.mapper.getStudentById"></collection>
    </resultMap>
           

student.mapper.xml

<mapper namespace="cn.sxt.entity.student.mapper">
    <select id="getStudentById"    resultType="Student">
        select * from  student where tid=#{id}
    </select>
</mapper>
           

使用

public static void main(String[] args) throws IOException {
        TeacherDao teacherDao=new TeacherDao();
        Teacher  teacher=teacherDao.getTeacher(1);
        System.out.println("teacher name="+teacher.getName());
        List<Student>  list=teacher.getStudents();
        for(Student stu:list){
            System.out.println("student name="+stu.getName());
        }
    }
           

查询嵌套原理，例析：

映射文件：

这种方式里的Minister属性mid、mname是通过查询得到的，而非定义包装的。

将查询到的mid、mname再传给Country类里的ministers集合。