Mybatis一對多的處理

表的設計和Mybatis多對一相同

表的設計 

利用Navicat for mysql建表，然後在檢視菜單點選ER圖表（概念模型） 如下： 

設計好的表ER圖表 

上述表 student tid設定外鍵參考teacher的id，是以建好表後兩者都為空要先添加teacher的資料。 

多個學生對于一個老師 

實體類 

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
}
           

編寫teacher.mapper.xml映射檔案

兩種處理方式 

第一種 結果嵌套

<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>
           

第二種 查詢嵌套 

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());
        }
    }
           

查詢嵌套原理，例析：

映射檔案：

這種方式裡的Minister屬性mid、mname是通過查詢得到的，而非定義包裝的。

将查詢到的mid、mname再傳給Country類裡的ministers集合。