MyBatis collection的两种形式——MyBatis学习笔记之九

<?xml version="1.0" encoding="utf8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<!--与以前一样，namespace的值是对应的映射器接口的完整名称-->
<mapper namespace="com.abc.mapper.TeacherMapper">
<!--TeacherMapper接口中getById方法对应的SQL语句。
查询教师及其指导的学生的信息。由于教师、学生都有
id、name、gender等属性，因此给教师的字段都起了别名-->
<select id="getById" parameterType="int" resultMap="supervisorResultMap">
select t.id t_id, t.name t_name, t.gender t_gender,
t.research_area t_research_area, t.title t_title,
s.id,s.name, s.gender,s.major,s.grade
from teacher t,student s where t.id=#{id}
and s.supervisor_id = t.id
</select>
<!--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"/>
<!--需要注意的是，上面的select语句中学生的字段名/别名应与
下面的column属性一致。ofType指collection包含的元素的类型，
此属性不可少-->
<collection property="supStudents"ofType="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"/>
<!--映射学生的指导教师属性，用到了
supervisorResultMap本身-->
<association property="supervisor"
resultMap="supervisorResultMap"/>
</collection>
</resultMap>
</mapper>

<?xml version="1.0" encoding="utf8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<!--与以前一样，namespace的值是对应的映射器接口的完整名称-->
<mapper namespace="com.abc.mapper.TeacherMapper">
<!--TeacherMapper接口中getById方法对应的SQL语句。
查询教师的信息。-->
<select id="getById" parameterType="int" resultMap="supervisorResultMap">
select * from teacher where id=#{id}
</select>
<!--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"/>
<!--ofType指collection包含的元素的类型，此属性不可少。
column属性指把上述的getById的select语句中的教师id列的值作为参数
传递给将要引用到的下述的getStudents的select语句，此属性不可少。
引用的形式为：命名空间.select语句id-->
<collection property="supStudents" column="id" ofType="Student"
select="com.abc.mapper.StudentMapper.getStudents"/>
</resultMap>
</mapper>

<?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">
<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"/>
<!--在这里引用supervisorResultMap和getById，
亦采用命名空间名.相关元素id的形式。
column="supervisor_id"属性不可少-->
<association property="supervisor"
resultMap="com.abc.mapper.TeacherMapper.supervisorResultMap"
select="com.abc.mapper.TeacherMapper.getById"
column="supervisor_id"/>
</resultMap>
<!--根据指导教师id查询学生信息-->
<select id="getStudents" parameterType="int"
resultMap="studentResultMap">
select * from student where supervisor_id = #{id}
</select>
</mapper>