mybatis返回HashMap结果类型与映射

最新推荐文章于 2023-05-11 07:27:56 发布

转载最新推荐文章于 2023-05-11 07:27:56 发布 · 3.7k 阅读

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本文深入探讨了MyBatis中高级结果映射的使用技巧，包括一对一和一对多关联的嵌套查询与结果，详细解释了如何通过设置避免N+1查询问题，并讨论了延迟加载的实现方式。

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<!--
一对一关联嵌套查询应用select属性
1)id="selectBlogAndRelatedAuthor" 中的 blog_author_id 对应到
<association property="author" column="blog_author_id"/>中的column="blog_author_id"

2)我们执行selectBlogAndRelatedAuthor 1次之后，所产生的每条记录都要再进行一个查询来获取author信息（N），
这就是N+1问题，应该使用延迟加载的方式，否则这有可能产生致命的后果。

3)对于一对一关联,我设置了
   <settings>
       <setting name="lazyLoadingEnabled" value="true"/>
   </settings>
却没有对Author进行延迟加载，不知为何。。。
-->
<resultMap id="blogResult" type="Blog" >
   <association property="author" column="blog_author_id" javaType="Author" select="selectAuthor" />
</resultMap>

<select id="selectBlogAndRelatedAuthor" parameterType="int" resultMap="blogResult" >
SELECT id,title,content,blog_author_id FROM blog WHERE id = #{id}
</select>

<select id="selectAuthor" parameterType="int" resultType="Author">
   SELECT id,author_name as "name" FROM author WHERE id = #{id}
</select>

<resultMap id="blogResult2" type="Blog">
   <id property="id" column="id" />
   <result property="title" column="title"/>
   <association property="author" column="blog_author_id" javaType="Author">
       <id property="id" column="author_id"/>
       <result property="name" column="author_name"/>
   </association>
</resultMap>
<select id="selectBlogAndRelatedAuthor2" parameterType="int" resultMap="blogResult2" >
   SELECT t.ID, t.TITLE, t.CONTENT, a.id as "author_id", a.author_name
   FROM blog t
   INNER JOIN author a ON t.BLOG_AUTHOR_ID = a.ID
   AND t.ID = #{id}
</select>

<resultMap type="Blog" id="blogResult3" >
   <id property="id" column="id"/>
   <result property="title" column="title"/>
   <result property="content" column="content"/>
   <collection property="posts" column="id" javaType="ArrayList" ofType="Post" select="selectPosts"/>
</resultMap>

<select id="selectBlogAndRelatedPosts" parameterType="int" resultMap="blogResult3" >
SELECT id, title, content FROM blog WHERE id = #{id}
</select>

<select id="selectPosts" parameterType="int" resultType="Post" >
SELECT p.id,p.subject,p.body as "postContent" FROM posts p WHERE p.blog_id =#{id}
</select>

<resultMap type="Blog" id="blogResultVersion" >
   <id property="id" column="id" />
   <result property="title" column="title"/>
   <result property="content" column="content"/>
   <collection property="posts" javaType="ArrayList" ofType="Post" column="id">
       <id property="id" column="post_id"/>
       <result property="subject" column="subject"/>
       <result property="postContent" column="postContent"/>
   </collection>
</resultMap>

<select id="selectBlogAndRelatedPosts2" parameterType="int" resultMap="blogResultVersion">
   SELECT t.id, t.title, t.content, p.id as "post_id", p.subject, p.BODY as "postContent"
   FROM blog t
   left outer JOIN posts p ON t.id = p.blog_id
   WHERE t.ID = #{id}
</select>