介绍

collection是封装一对多关系的，通常情况下是一个列表，association是一对一，通常情况是一个对象。例如：查询班级下所有的学生，一个班级可以有多个学生，这就是一对多。

案例

有一个学生表，一个班级表，需求根据班级表查出该班级下所有的学生

一对多查询

SELECT
s.name studentsName ,
s.classes,
s.id studentsId,
c.id ,
c.name FROM classes c 
JOIN students s ON c.id=2 AND c.id=s.classes

实体类

学生实体

@Data
public class Students {
    Long id;
    String name;
    Long classes;

    //班级消息
    Classes belong;
}

班级实体

@Data
public class Classes {
    Long id;
    String name;
    List<Students> students;
}

controller

@RestController
@RequestMapping("/classes")
public class ClassesController {

    @Autowired
    ClassesMapper classesMapper;
    
    @GetMapping("/getClass")
    public Result getClass(Integer id){
        return  Result.success("获取成功",classesMapper.list(id););
    }
    
}

Mapper

@Mapper
public interface ClassesMapper {
    Classes list(Integer id);
}

<select id="list" resultMap="studentList" >
    SELECT   s.name studentsName ,
             s.classes,
             s.id studentsId,
             c.id ,
             c.name FROM classes c JOIN students s ON c.id=#{id} AND c.id=s.classes
</select>

    <resultMap id="studentList" type="com.example.demo.demos.web.pojo.Classes" >
        <id column="id" property="id"></id>
        <result column="name" property="name"></result>
        
        <collection property="students" ofType="com.example.demo.demos.web.pojo.Students" >
    
            <id property="id" column="studentsId"></id>
            <result property="name" column="studentsName"></result>            
            <result property="classes" column="classes" ></result>
            
        </collection>
        
    </resultMap>

查询结果

{
    "code": 200,
    "msg": "获取成功",
    "data": {
        "id": 3,
        "name": "大数据2",
        "students": [
            {
                "id": 2,
                "name": "赵子龙",
                "classes": 3,
                "belong": null
            },
            {
                "id": 4,
                "name": "马冬梅",
                "classes": 3,
                "belong": null
            },
            {
                "id": 5,
                "name": "易烊干玺",
                "classes": 3,
                "belong": null
            }
        ]
    }
}