MyBatis级联操作

先解决一下idea无法识别lombok构造方法的问题，解决方案是在idea的插件中下载并安装lombok插件。

MyBatis级联操作，列举最简单的student-classes（学生与班级）的关系表：

create table if not exists student (
    id int primary key auto_increment,
    name varchar(20) not null comment '学生姓名',
    cid int not null comment '班级id'
);

create table if not exists classes (
    id int primary key auto_increment,
    name varchar(20) not null comment '班级名'
);

接下来，创建关系表对应的实体类：

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

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

在repository包下新建StudentRepository接口：

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

然后创建对应的mapper文件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">

<mapper namespace="com.wts.repository.StudentRepository">
    
    <resultMap id="studentMap" type="com.wts.entity.Student">
        <id column="id" property="id"></id>
        <result column="name" property="name"></result>
        <association property="classes" javaType="com.wts.entity.Classes">
            <id column="cid" property="id"></id>
            <result column="cname" property="name"></result>
        </association>
    </resultMap>
    
    <select id="findById" parameterType="long" 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>
</mapper>

注意这里有几个限制：

1.命名空间，xml文件的namespace必须是对应接口的全类名

2.Statement标签的id必须与接口方法相同，其中parameterType为参数，resultType为返回类型，复杂类型用resultMap

3.复杂类型resultMap中多对一用association，一堆多用集合collection

MyBatis执行sql返回的结果集会和关系对象映射起来，注意列与字段的对应关系。

然后将mapper引入：

<mappers>
    <mapper resource="com/wts/repository/StudentRepository.xml"></mapper>
</mappers>

编写测试方法：

@Test
public void test03() {
    InputStream inputStream = AppTest.class.getClassLoader().getResourceAsStream("config.xml");
    SqlSessionFactory sqlSessionFactory = (new SqlSessionFactoryBuilder()).build(inputStream);
    try (SqlSession sqlSession = sqlSessionFactory.openSession()) {
        // 级联查询
        StudentRepository studentRepository = sqlSession.getMapper(StudentRepository.class);
        System.out.println(studentRepository.findById(1L));
    }
}