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expected one result (or null) to be returned by selectone(), but found: 5

发布时间 2023-05-31 00:17:13作者: Himmelbleu

以下是我的 mapper 文件的内容，是一个一对多查询。但是查询结果的 Book 映射不是一个，并且不能映射正确。

<?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.bookstore.mapper.BookMapper">

    <resultMap id="resultOfQueryBookById" type="Book" autoMapping="true">
        <collection property="tags" ofType="BookTag" columnPrefix="bt_" autoMapping="true"/>
        <collection property="authors" ofType="BookAuthor" columnPrefix="ba_" autoMapping="true"/>
        <collection property="comments" ofType="BookComment" columnPrefix="bc_" autoMapping="true"/>
        <collection property="previews" ofType="BookPreview" columnPrefix="bp_" autoMapping="true"/>
    </resultMap>

    <select id="queryBookById" resultMap="resultOfQueryBookById">
        SELECT b.*,
               bc.id        bc_id,
               bc.content   bc_content,
               bc.digg      bc_digg,
               bc.bury      bc_bury,
               bc.type      bc_type,
               bc.post_date bc_post_date,
               bt.tag       bt_tag,
               ba.author    ba_author,
               bp.url       bp_url
        FROM books b
                 INNER JOIN book_tags bt ON bt.book_id = b.id
                 INNER JOIN book_authors ba ON ba.book_id = b.id
                 INNER JOIN book_comments bc ON bc.book_id = b.id
                 INNER JOIN book_previews bp ON bp.book_id = b.id
        WHERE b.id = #{id};
    </select>
</mapper>

我开启了 resultMap 自动映射，所以 collection 和 resultMap 第一层都没有任何子标签。但问题就出在这里，就是因为我的主表，即 Book 没有手动添加 id 的 result 子标签导致查询结果不能正确映射。

因此，在开启自动映射的情况下，有一对多或者多对一，主表的实体类 Book 必须包含一个 id 字段（主键）的 result 子标签。

<resultMap id="resultOfQueryBookById" type="Book" autoMapping="true">
    <result property="id" column="id"/>
    <collection property="tags" ofType="BookTag" columnPrefix="bt_" autoMapping="true"/>
    <collection property="authors" ofType="BookAuthor" columnPrefix="ba_" autoMapping="true"/>
    <collection property="comments" ofType="BookComment" columnPrefix="bc_" autoMapping="true"/>
    <collection property="previews" ofType="BookPreview" columnPrefix="bp_" autoMapping="true"/>
</resultMap>