mybatis主子表嵌套查询，返回一个对象带有集合属性

前段时间在工作中遇到需要一对多关联查询的情况，百度了好久遇到各种坑，所以在此记录一下，话不多说直接开始

首先，先建几个测试类，主子表通过grade_id关联，具体属性注释我就省略不写了，大家能看懂

• 主表

	@Data
	@TableName("grade")
	public class Grade implements Serializable {
	    private Long id;
	    private String name;
	    private String address;
		}

• 子表1

  @Data
  @TableName("teacher")
  public class Teacher implements Serializable {
      private Long id;
      private String name;
      private Integer age;
      private String categroy;
      private Integer level;
      private Long gradeId;		// 关联主表id
  	}
  

• 子表2

  @Data
  @TableName("student")
  public class Student implements Serializable {
      private Long id;
      private String name;
      private Integer age;
      private String address;
      private Long gradeId;		// 关联主表id
  }
  

• 表结构和实体类一样我就不放了，没啥特别的

其次，创建一个DTO用来接收查询出来的结果，这个大家都懂，不解释

@Data
public class GradeDTO implements Serializable {
    private Long id;
    private String name;
    private String address;
    private List<Teacher> teacherList;
    private List<Student> studentList;
}

重点，配置mapper.xml文件

• 首先需要配置resultMap用来接收返回结果，这个大家百度得到，关联集合就是用collection，如下

      <resultMap type="com.kasi.mybatis_plus.model.GradeDTO" id="gradeMap">
      <id column="id" property="id"/>
      <result column="name" property="name"/>
      <result column="address" property="address"/>
      <collection property="teacherList" ofType="com.kasi.mybatis_plus.model.Teacher" >
          <id column="t_id" property="id"/>
          <result column="t_name" property="name"/>
          <result column="t_age" property="age"/>
          <result column="t_categroy" property="categroy"/>
          <result column="t_level" property="level"/>
      </collection>
      <collection property="studentList" ofType="com.kasi.mybatis_plus.model.Student">
          <id column="s_id" property="id"/>
          <result column="s_name" property="name"/>
          <result column="s_age" property="age"/>
          <result column="s_address" property="address"/>
      </collection>
  </resultMap>
  

  常用的属性就不说了，特别要注意的是ofType这个要指向各自表的实体类，然后子表的column属性不要和主表重名，一旦重名就会出问题，不知道是不是我配置的问题，希望有大佬指教，反正不重名就可以
• 接下来就是查询语句了，很简单，只要别名和上面resultMap对的上就ok

      <select id="get" resultMap="gradeMap">
      SELECT
          a.id,
          a.name,
          a.address,
          b.id as t_id,
          b.name as t_name,
          b.age as t_age,
          b.categroy as t_categroy,
          b.level as t_level,
          c.id as s_id,
          c.name as s_name,
          c.age as s_age,
          c.address as s_address
      FROM grade a
      LEFT JOIN teacher b ON a.id=b.grade_id
      LEFT JOIN student c ON a.id=c.grade_id
      WHERE a.id=#{id}
  </select>
  

  最后，看看查询结果，其实很简单的，网上教程也很多，但是别名那个地方很多人都没有强调导致犯了很多错

  ps：查询集合也是一样的，不说了
  • 数据表查询
    
  • 实际查询结果

    {
    "id": 1,
    "name": "3年1班",
    "address": "三楼",
    "teacherList": [
        {
            "id": 1,
            "name": "宋元桥",
            "age": 50,
            "categroy": "武当",
            "level": 1,
            "gradeId": null
        },
        {
            "id": 2,
            "name": "俞莲舟",
            "age": 43,
            "categroy": "武当",
            "level": 2,
            "gradeId": null
        }
    ],
    "studentList": [
        {
            "id": 1,
            "name": "金毛狮王",
            "age": 47,
            "address": "冰火岛",
            "gradeId": null
        },
        {
            "id": 2,
            "name": "紫衫龙王",
            "age": 39,
            "address": "灵蛇岛",
            "gradeId": null
        }
    ]
    

  }