一、嵌套的resultMap
这种方法本质上就是上篇博文介绍的方法，只是把教师实体映射从association元素中提取出来，用一个resultMap元素表示。然后association元素再引用这个resultMap元素。修改上篇博文示例的StudentMapper.xml如下：
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<?
xml
version
=
"1.0"
encoding
=
"utf8"
?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<
mapper
namespace
=
"com.abc.mapper.StudentMapper"
>
<
select
id
=
"getById"
parameterType
=
"int"
resultMap
=
"studentResultMap"
>
select s.id s_id,
s.name s_name,
s.gender s_gender,
s.major s_major,
s.grade s_grade,
t.id t_id,
t.name t_name,
t.gender t_gender,
t.title t_title,
t.research_area t_research_area
from student s left join teacher t
on s.supervisor_id = t.id
where s.id=#{id}
</
select
>
<
resultMap
id
=
"studentResultMap"
type
=
"Student"
>
<
id
property
=
"id"
column
=
"s_id"
/>
<
result
property
=
"name"
column
=
"s_name"
/>
<
result
property
=
"gender"
column
=
"s_gender"
/>
<
result
property
=
"major"
column
=
"s_major"
/>
<
result
property
=
"grade"
column
=
"s_grade"
/>
<!--使用resultMap属性引用下面的教师实体映射-->
<
association
property
=
"supervisor"
javaType
=
"Teacher"
resultMap
=
"supervisorResultMap"
/>
</
resultMap
>
<!--教师实体映射-->
<
resultMap
id
=
"supervisorResultMap"
type
=
"Teacher"
>
<
id
property
=
"id"
column
=
"t_id"
/>
<
result
property
=
"name"
column
=
"t_name"
/>
<
result
property
=
"gender"
column
=
"t_gender"
/>
<
result
property
=
"researchArea"
column
=
"t_research_area"
/>
<
result
property
=
"title"
column
=
"t_title"
/>
</
resultMap
>
</
mapper
>
|
在这里，为了让select语句更加清晰可读，每个字段都使用了别名。association元素的属性javaType="Teacher"可以不要，不像上篇博文示例那样，没有此属性即报错。这大概是因为引用的resultMap元素已经指明了映射的类型为“Teacher”。另外，今天偶然发现，把StudentMapper.xml的encoding属性由“UTF-8”改为“utf8”即可添加中文注释。
运行结果如下：
这种方式的好处是此resultMap元素可在其它地方被重用（可在博文下方附件下载处下载示例源码）。
二、嵌套的select语句
这种方式是使用一条单独的select语句来加载关联的实体（在本例中就是教师实体），然后在association元素中引用此select语句（注：此方法会产生N+1问题，关于这个问题可参考本系列博客中的“MyBatis中的N+1问题”）。修改StudentMapper.xml如下：
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<?
xml
version
=
"1.0"
encoding
=
"utf8"
?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<
mapper
namespace
=
"com.abc.mapper.StudentMapper"
>
<
select
id
=
"getById"
parameterType
=
"int"
resultMap
=
"studentResultMap"
>
select id,name,gender,major,grade,supervisor_id from student where id=#{id}
</
select
>
<
select
id
=
"selectSupervisor"
parameterType
=
"int"
resultMap
=
"supervisorResultMap"
>
select id,name,gender,title,research_area
from teacher where id=#{id}
</
select
>
<
resultMap
id
=
"studentResultMap"
type
=
"Student"
>
<
id
property
=
"id"
column
=
"id"
/>
<
result
property
=
"name"
column
=
"name"
/>
<
result
property
=
"gender"
column
=
"gender"
/>
<
result
property
=
"major"
column
=
"major"
/>
<
result
property
=
"grade"
column
=
"grade"
/>
<!--column="supervisor_id"不能少。此列的值作为参数
传递给要引用的select语句，用来查询相应学生的指导教师
的信息。select属性指定要引用的select语句-->
<
association
property
=
"supervisor"
javaType
=
"Teacher"
column
=
"supervisor_id"
select
=
"selectSupervisor"
/>
</
resultMap
>
<!--教师实体映射-->
<
resultMap
id
=
"supervisorResultMap"
type
=
"Teacher"
>
<
id
property
=
"id"
column
=
"id"
/>
<
result
property
=
"name"
column
=
"name"
/>
<
result
property
=
"gender"
column
=
"gender"
/>
<
result
property
=
"researchArea"
column
=
"research_area"
/>
<
result
property
=
"title"
column
=
"title"
/>
</
resultMap
>
</
mapper
>
|
association元素的属性javaType="Teacher"也可以不要，这大概也是因为引用的select语句中已经指明了要用到的resultMap。
执行结果如下（可在博文下方附件下载处下载示例源码）：
本文转自 NashMaster2011 51CTO博客，原文链接：http://blog.51cto.com/legend2011/940072，如需转载请自行联系原作者
