MyBatis一对多的处理

环境搭建

1.实体类：

package com.kuang.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<Student> students;
}

2.编写接口：

package com.kuang.dao;
import com.kuang.pojo.Teacher;
import org.apache.ibatis.annotations.Param;
import java.util.List;
public interface TeacherMapper {
   //获取指定老师下的所有学生及老师信息
   Teacher getTeacher(@Param("tid") int id);
}

按照结果嵌套查询

<?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.kuang.dao.TeacherMapper">
    <select id="getTeacher" resultMap="TeacherStudent">
        select s.id sid,s.name sname,t.name tname,t.id tid
        from student s,teacher t
        where s.tid=t.id and t.id=#{tid}
    </select>
    <resultMap id="TeacherStudent" type="Teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <collection property="students" ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="tname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>
</mapper>

测试：

import com.kuang.dao.TeacherMapper;
import com.kuang.pojo.Teacher;
import com.kuang.utils.MyBatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import java.util.List;
public class MyTest {
    @Test
    public void getTeachertest(){
        SqlSession sqlSession = MyBatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacher(1);
        System.out.println(teacher);
        sqlSession.close();
    }
}

按照子查询嵌套查询

<select id="getTeacher2" resultMap="TeacherStudent2">
    select * from mybatis.teacher where id=#{tid}
</select>
<resultMap id="TeacherStudent2" type="Teacher">
    <result property="id" column="id"/>
    <result property="name" column="name"/>
    <collection property="students" column="id"
                javaType="ArrayList" ofType="Student"
                select="getStudentByTeacherId"/>
</resultMap>
<select id="getStudentByTeacherId" resultType="Student">
    select  * from student where tid=#{id}
</select>

测试：

import com.kuang.dao.TeacherMapper;
import com.kuang.pojo.Teacher;
import com.kuang.utils.MyBatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

public class MyTest {
    @Test
    public void getTeachertest2(){
        SqlSession sqlSession = MyBatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacher(1);
        System.out.println(teacher);
        sqlSession.close();
    }
}

小结：
1.关联：association【多对一】
2.集合：collection【一对多】
3.javaType：用来指定实体类中属性的类型
4.ofType：用来指定映射到List或者集合中的类类型，即泛型中的约束类型