Mybatis 映射 - Corn1ng's Blog

<!-- Very Complex Result Map -->  
<resultMap id="detailedBlogResultMap" type="Blog">  
<constructor>  
<idArg column="blog_id" javaType="int"/>  
</constructor>  
<result property="title" column="blog_title"/>  
<association property="author" column="blog_author_id" javaType=" Author">  
<id property="id" column="author_id"/>  
<result property="username" column="author_username"/>  
<result property="password" column="author_password"/>  
<result property="email" column="author_email"/>  
<result property="bio" column="author_bio"/>  
<result property="favouriteSection" column="author_favourite_section"/>  
</association>  
<collection property="posts" ofType="Post">  
<id property="id" column="post_id"/>  
<result property="subject" column="post_subject"/>  
<association property="author" column="post_author_id" javaType="Author"/>  
<collection property="comments" column="post_id" ofType=" Comment">  
<id property="id" column="comment_id"/>  
</collection>  
<collection property="tags" column="post_id" ofType=" Tag" >  
<id property="id" column="tag_id"/>  
</collection>  
<discriminator javaType="int" column="draft">  
<case value="1" resultType="DraftPost"/>  
</discriminator>  
</collection>  
</resultMap> <wbr>

resultmap

constructor 实例化的时候通过构造器将结果集注入类中
ID 结果集ID 方便全局调用
result 注入一个字段或者Javabean属性的结果。
association 一对一嵌套
collection 一对多嵌套
discriminator 使用一个结果值以决定使用哪个resultMap

Id result 元素

1 2	<id property="id" column="post_id"/> <result property="subject" column="post_subject"/>

Association 元素

<association property="author" column="blog_author_id" javaType=" Author">
<id property="id" column="author_id"/>
<result property="username" column="author_username"/>
</association>

association 元素处理一对一类型关系这时需要告诉mybatis 如何加载一个联合查询，mybatis使用两种方式加载1 通过执行另一个返回预期复杂类型的映射SQL语句2.通过嵌套结果映射来处理连接结果集的重复子集。

1<resultMap id=”blogResult” type=”Blog”>  
<association property="author" column="blog_author_id" javaType="Author"  
select=”selectAuthor”/>  
</resultMap>   
   
<select id=”selectAuthor” parameterType=”int” resultType="Author">  
SELECT * FROM AUTHOR WHERE ID = #{id}  
</select>

<resultMap id="blogResult" type="Blog">  
<id property=”blog_id” column="id" />  
<result property="title" column="blog_title"/>  
<association property="author" column="blog_author_id" javaType="Author"  
resultMap=”authorResult”/>  
</resultMap>  
   
<resultMap id="authorResult" type="Author">  
<id property="id" column="author_id"/>  
<result property="username" column="author_username"/>  
<result property="password" column="author_password"/>  
<result property="email" column="author_email"/>  
<result property="bio" column="author_bio"/>  
</resultMap>

也可以直接把id 标签 result 标签写到里面。

collection 元素

<collection property="posts" ofType="domain.blog.Post">  
<id property="id" column="post_id"/>  
<result property="subject" column="post_subject"/>  
<result property="body" column="post_body"/>  
</collection>

<resultMap id=”blogResult” type=”Blog”>  
	<collection property="posts" javaType=”ArrayList” 			column="blog_id"  ofType="Post" 					  		select=”selectPostsForBlog”/>  
</resultMap>  
<select id=”selectBlog” parameterType=”int” resultMap=”blogResult”>  
SELECT * FROM BLOG WHERE ID = #{id}  
</select>  
<select id=”selectPostsForBlog” parameterType=”int” resultType="Author">  
SELECT * FROM POST WHERE BLOG_ID = #{id}  
</select>

这时会有一个新的属性oftype 这个元素是用来区别JavaBean属性类型和集合类型所包括的类型。

Contents

resultmap