Mybatis级联查询-associatio&和collection (多对一&一对多)

一对多和多对一

Student

package com.southwind.entity;
import lombok.Data;
@Data
public class Student {
    private long id;
    private String name;
    private Classes classes;
}

Classes

package com.southwind.entity;
import lombok.Data;
import java.util.List;
@Data
public class Classes {
    private long id;
    private String name;
    private List<Student> students;
}

多对一:

StudentRepository

package com.southwind.repository;
import com.southwind.entity.Student;
public interface StudentRepository {
    public Student findById(long id);
}

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.southwind.repository.StudentRepository">
    <resultMap id="studentMap" type="com.southwind.entity.Student">
        <id column="id" property="id"></id>
        <result column="name" property="name"></result>
        <association property="classes" javaType="com.southwind.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>

type是查询的主类所在的位置,id是主键,result是其他属性值,column是sql语句中的字段,property是实体类中映射的属性名,association表示的是关联,javaType是所在的类,collection表示的是集合,ofType是集合中元素的类型

一对多

ClassesRepository

package com.southwind.repository;
import com.southwind.entity.Classes;
public interface ClassesRepository {
    public Classes findById(long id);
}

ClassesRepository.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.southwind.repository.ClassesRepository">
    <resultMap id="classesMap" type="com.southwind.entity.Classes">
        <id column="cid" property="id"></id>
        <result column="cname" property="name"></result>
     	<!-- property: 指的是集合属性的值, ofType：指的是集合中元素的类型 -->
        <collection property="students" ofType="com.southwind.entity.Student">
            <id column="id" property="id"/>
            <result column="name" property="name"/>
        </collection>
    </resultMap>
    
    <select id="findById" parameterType="long" 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>