一对多处理

一、搭建环境

沿用上一篇博客“多对一处理”项目

1.1、修改实体类Students、Teacher

package com.massimo.pojo;

import lombok.Data;

@Data
public class Students {
    private int id;
    private String name;
    private int tid;
}

package com.massimo.pojo;

import lombok.Data;

import java.util.List;

@Data
public class Teacher {
    private int id;
    private String name;

    //一个老师拥有多个学生
    private List<Students> students;
}

二、测试

## 2.1、TeacherMapper接口

package com.massimo.dao;

import com.massimo.pojo.Teacher;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;

import java.util.List;

public interface TeacherMapper {
   List<Teacher> getTeacher();
   
}

2.2、TeacherMapper.xml

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

<mapper namespace="com.massimo.dao.TeacherMapper">

    <select id="getTeacher" resultType="Teacher">
        select * from teacher;
    </select>

</mapper>

2.3、MyTest测试类

@Test
    public void test(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        List<Teacher> teacherList = mapper.getTeacher();
        for (Teacher teacher : teacherList){
            System.out.println(teacher);
        }
        sqlSession.close();
    }

2.4、效果

三、按照结果嵌套处理

3.1、TeacherMapper接口

//获取指定老师的所有信息及其下所有学生的信息
    Teacher getTeacher(@Param("tid") int id);

3.2、TeacherMapper.xml

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

<mapper namespace="com.massimo.dao.TeacherMapper">

    <!--按结果嵌套查询-->
    <select id="getTeacher" resultMap="TeacherStudents">
        select s.id sid , s.name sname , t.name tname , t.id tid
        from students s , teacher t
        where s.tid = t.tid and t.id = #{tid}
    </select>

    <resultMap id="TeacherStudents" type="Teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
    <!-- 集合 ： collection
          javaType = “” 指定属性的类型
          集合中的泛型信息，我们使用ofType获取
    -->
        <collection property="students" ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>
</mapper>

3.3、MyTest测试类

    @Test
    public void test(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacher(1);
        System.out.println(teacher);
        sqlSession.close();
    }

3.4、结果

四、按照查询嵌套处理

4.1、TeacherMapper接口

    Teacher getTeacher2(@Param("tid") int id);

4.2、TeacherMapper.xml

<select id="getTeacher2" resultMap="TeacherStudents2">
        select * from teacher where id = #{tid};
    </select>

    <resultMap id="TeacherStudents2" type="Teacher">
        <collection property="students" javaType="ArrayList" ofType="Students" select="getStudentsByTeacherId" column="id"/>
    </resultMap>

    <select id="getStudentsByTeacherId" resultType="Students">
        select * from  students where tid = #{tid}
    </select>

4.3、MyTest测试类

    @Test
    public void test2(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher2 = mapper.getTeacher2(1);
        System.out.println(teacher2);
        sqlSession.close();
    }

4.4、结果

五、小结

• 关联—association 【多对一】

• 集合—collection 【一对多】

• javaType & ofType

• javaType 用来指定实体类中属性的类型

• ofType 用来指定映射到到List或者集合中的pojo类型，泛型中的约束