myBatis：级联查询

数据库mybatis名字而已，和mybatis框架毫无关联下有两张表classes和student关联，下面分别从一对多和多对一在mybatis框架下进行关联查询。不管是一对多还是多对一，都需要在.xml文件中配置resultMap。

• 新建classes类和student类

package com.lmybatis.entity;

import lombok.Data;

import java.util.List;

@Data
public class Classes {
    private int id;
    private String name;
    private List<Student> students;
}

package com.lmybatis.entity;

import lombok.Data;

@Data
public class Student {
    private int id;
    private String name;
    private Classes classes;
}

• 为Classes类创建接口ClassRepository，为Student类创建接口StudentRepository

package com.lmybatis.repository;

import com.lmybatis.entity.Account;
import com.lmybatis.entity.Classes;

import java.util.List;

public interface ClassesRepository {
    public int save(Classes classes);
    public int update(Classes classes);
    public int deleteById(int id);  //删除
    public List<Classes> findAll(); //查询
    public Classes findById(int id);//查询
}

package com.lmybatis.repository;

import com.lmybatis.entity.Student;

public interface StudentRepository {
    public Student findById(int id);
}

• 配置ClassesRepository.xml和StudentRepository.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<!--namespace中放入接口路径-->
<mapper namespace="com.lmybatis.repository.ClassesRepository">
    <resultMap id="classesMap" type="com.lmybatis.entity.Classes">
        <id column="cid" property="id"></id>
        <result column="cname" property="name"></result>
        <collection property="students" ofType="com.lmybatis.entity.Student">
            <id column="id" property="id"></id>
            <result column="name" property="name"></result>
        </collection>
    </resultMap>
    <select id="findById" parameterType="int" resultMap="classesMap">
        select s.id,s.name,c.id as cid,c.name as cname from student s,classes c where c.id = #{id} and s.cid = c.id
    </select>
</mapper>
        <!--记住！要在config.xml中配置该xml文件-->

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.lmybatis.repository.StudentRepository">

    <resultMap id="studentMap" type="com.lmybatis.entity.Student">
        <id column="id" property="id"></id>
        <result column="name" property="name"></result>
        <association property="classes" javaType="com.lmybatis.entity.Classes">
            <id column="cid" property="id"></id>
            <result column="cname" property="name"></result>
        </association>
    </resultMap>

    <select id="findById" parameterType="int" resultMap="studentMap">
        select s.id,s.name,c.id as cid,c.name as cname from student s,classes c where s.id = #{id} and s.cid = c.id
    </select>


    <resultMap id="studentMapLazy" type="com.lmybatis.entity.Student">
        <id column="id" property="id"></id>
        <result column="name" property="name"></result>
        <association property="classes" javaType="com.lmybatis.entity.Classes" select="com.lmybatis.repository.ClassesRepository.findByIdLazy" column="cid"></association>
    </resultMap>

    <select id="findByIdLazy" parameterType="int" resultMap="studentMapLazy">
        select * from student where id = #{id}
    </select>

</mapper>

•  测试，在classes表中寻找在id = 1的班级的所有人。明显，输出应为张三和李四。

package com.lmybatis.test;

import com.lmybatis.entity.Account;
import com.lmybatis.entity.Classes;
import com.lmybatis.repository.AccountRepository;
import com.lmybatis.repository.ClassesRepository;
import com.lmybatis.repository.StudentRepository;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.InputStream;
import java.util.List;

public class test2 {
    public static void main(String[] args) {
        //mybatis核心接口和类
        InputStream inputStream = test1.class.getClassLoader().getResourceAsStream("config.xml");
        SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
        SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();

//        StudentRepository studentRepository = sqlSession.getMapper(StudentRepository.class);
//        System.out.println(studentRepository.findById(1));

        ClassesRepository classesRepository = sqlSession.getMapper(ClassesRepository.class);
        System.out.println(classesRepository.findById(1));
    }
}

• 输出