Mybatis联表查询：多对多（注解实现）_mybatis 返回多个关联对象-优快云博客

本文链接：https://blog.youkuaiyun.com/wang9820/article/details/122115154

本文详细介绍了使用Mybatis的注解方式处理多对多关系的联表查询，包括数据库表结构的设计，自定义结果类CommentWithTag的封装，以及CommentMapper和TagMapper的注解配置。在测试类中，要注意返回结果类需添加特定注解以避免查询错误。

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1、数据库表结构

在这里插入图片描述

2、返回结果类封装

CommentWithTag .java

@Getter
@Setter
@ToString
@Builder
@AllArgsConstructor
@NoArgsConstructor
@JsonIgnoreProperties(value = {"handler"})
public class CommentWithTag implements Serializable {
    /**
     * 编号
     */
    @ApiModelProperty(value="编号")
    private Integer id;

    /**
     * 图片，最多5张 [aa.jpg,bb.jpg,cc.jpg]
     */
    @ApiModelProperty(value="图片，最多5张 [aa.jpg,bb.jpg,cc.jpg]")
    private String pics;

    /**
     * 评论内容
     */
    @ApiModelProperty(value="评论内容")
    private String content;

    /**
     * 星级：1~5
     */
    @ApiModelProperty(value="星级：1~5")
    private Integer stars;

    /**
     * 其他用户对这条评论的评论个数
     */
    @ApiModelProperty(value="其他用户对这条评论的评论个数")
    private Integer comments;

    /**
     * 其他用户对这条评论的点赞个数
     */
    @ApiModelProperty(value="其他用户对这条评论的点赞个数")
    private Integer agrees;

    /**
     * 上一级评论编号，如果为null表示是购买者评论
     */
    @ApiModelProperty(value="上一级评论编号，如果为null表示是购买者评论")
    private Integer pid;

    /**
     * 评论时间
     */
    @TableField(value = "create_time")
    @ApiModelProperty(value="评论时间")
    private LocalDateTime createTime;

    /**
     * 状态
     */
    @TableField(value = "`status`")
    @ApiModelProperty(value="状态")
    private Integer status;

    /**
     * 标签内容列表
     */
    @ApiModelProperty(value="标签内容列表")
    private List<String> titleList;

}

3、mapper层注解

CommentMapper

@Mapper
public interface CommentMapper extends BaseMapper<Comment> {

    @Select("select * from tb_comment where user_id = #{userId}")
    @Results({
            @Result(id = true,column = "id",property = "id"),
            @Result(column = "pics",property = "pics"),
            @Result(column = "content",property = "content"),
            @Result(column = "stars",property = "stars"),
            @Result(column = "comments",property = "comments"),
            @Result(column = "agrees",property = "agrees"),
            @Result(column = "pid",property = "pid"),
            @Result(column = "createTime",property = "create_time"),
            @Result(column = "status",property = "status"),
            @Result(
                    column = "id",property = "titleList",
                    many = @Many(select = "com.hc.mapper.TagMapper.selectTagByCommentId",fetchType = FetchType.LAZY )
            )
    })
    List<CommentWithTag> selectCommentWithTagByUserId(Long userId);
}

TagMapper

@Mapper
public interface TagMapper extends BaseMapper<Tag> {

    @Select("select * from tb_tag where id in (select tag_id from tb_comment_tag where comment_id = #{commentId})")
    List<Tag> selectTagByCommentId(Integer commentId);
}

4、编写测试类

@SpringBootTest
class CommentMapperTest {

    @Resource
    private CommentMapper commentMapper;

    @Test
    void selectCommentWithTagByUserId(){
        List<CommentWithTag> commentWithTags = commentMapper.selectCommentWithTagByUserId(220L);
        System.out.println(JsonUtil.obj2String(commentWithTags));
    }

}

注意

返回的结果类必须加下面这行注解

@JsonIgnoreProperties(value = {"handler"})

否则产生错误信息

No serializer found for class org.apache.ibatis.executor.loader.javassist
.JavassistProxyFactory$EnhancedResultObjectProxyImpl 
and no properties discovered to create BeanSerializer 
(to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS)
 (through reference chain: java.util.ArrayList[0]
 ->com.hc.res.CommentWithTag_$$_jvst497_0["handler"])