mybatis主子表嵌套查詢，返回一個對象帶有集合屬性

前段時間在工作中遇到需要一對多關聯查詢的情況，百度了好久遇到各種坑，所以在此記錄一下，話不多說直接開始

首先，先建幾個測試類，主子表通過grade_id關聯，具體屬性註釋我就省略不寫了，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;		// 關聯主表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;		// 關聯主表id
  }
  

• 表結構和實體類一樣我就不放了，沒啥特別的

其次，創建一個DTO用來接收查詢出來的結果，這個大家都懂，不解釋

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

重點，配置mapper.xml文件

• 首先需要配置resultMap用來接收返回結果，這個大家百度得到，關聯集合就是用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>
  

  常用的屬性就不說了，特別要注意的是ofType這個要指向各自表的實體類，然後子表的column屬性不要和主表重名，一旦重名就會出問題，不知道是不是我配置的問題，希望有大佬指教，反正不重名就可以
• 接下來就是查詢語句了，很簡單，只要別名和上面resultMap對的上就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>
  

  最後，看看查詢結果，其實很簡單的，網上教程也很多，但是別名那個地方很多人都沒有強調導致犯了很多錯

  ps：查詢集合也是一樣的，不說了
  • 數據表查詢
    
  • 實際查詢結果

    {
    "id": 1,
    "name": "3年1班",
    "address": "三樓",
    "teacherList": [
        {
            "id": 1,
            "name": "宋元橋",
            "age": 50,
            "categroy": "武當",
            "level": 1,
            "gradeId": null
        },
        {
            "id": 2,
            "name": "俞蓮舟",
            "age": 43,
            "categroy": "武當",
            "level": 2,
            "gradeId": null
        }
    ],
    "studentList": [
        {
            "id": 1,
            "name": "金毛獅王",
            "age": 47,
            "address": "冰火島",
            "gradeId": null
        },
        {
            "id": 2,
            "name": "紫衫龍王",
            "age": 39,
            "address": "靈蛇島",
            "gradeId": null
        }
    ]
    

  }