多对一处理 和一对多处理的处理
在前面创建的两个表中,学生和老师的关系,学生对于老师是多对一的关系,老师对于学生是一对多的关系。在MyBatis中处理这样的相对复杂的关系需要用到对象(association)和 集合(collection)。
多对一
实体类
public class Student2 { private int id; private String name; //学生和老师是多对一的关系,需要在学生中关联老师 private Teacher2 teacher; } public class Teacher2 { private int id; private String name; }
接口
public interface Student2Mapper { //获取所有学生信息以及他的老师 List<Student2> getStudent(); List<Student2> getStudent2(); }
xml配置
<!--按照结果嵌套处理--> <select id="getStudent" resultMap="StudentTeacher"> select s.id sid,s.name sname,t.name tname,t.id tid from student s,teacher t where s.tid = t.id </select> <resultMap id="StudentTeacher" type="Student2"> <result property="id" column="sid"/> <result property="name" column="sname"/> <association property="teacher" javaType="Teacher2"> <result property="id" column="tid"/> <result property="name" column="tname"/> </association> </resultMap> <!--通过子查询的方法--> <select id="getStudent2" resultMap="StudentTeacher2"> select * from student </select> <resultMap id="StudentTeacher2" type="Student2"> <!-- 复杂的属性需要单独处理 对象:association 集合:collection --> <association property="teacher" column="tid" javaType="Teacher2" select="getTeacher"/> </resultMap> <select id="getTeacher" resultType="Teacher2"> select * from teacher where id = #{tid} </select>
一对多
这是从老师的角度,实体类如下
public class Student {
private int id;
private String name;
private int tid;
}
public class Teacher {
private int id;
private String name;
private List<Student> students;
}
List<Teacher> getTeacher2(@Param("tid") int id);
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.hudu.dao.TeacherMapper">
<select id="getTeacher" resultType="Teacher">
select * from teacher
</select>
<!--按照结果嵌套-->
<select id="getTeacher2" resultMap="TeacherStudent">
SELECT s.id sid,s.name sname,t.name tname,t.id tid
FROM teacher t,student s
where t.id = s.tid and t.id = #{tid}
</select>
<resultMap id="TeacherStudent" type="Teacher">
<result property="id" column="tid"/>
<result property="name" column="tname"/>
<!--复杂的属性,我们需要单独处理 对象:association 集合:collection
javaType=""指定属性的类型
集合中的泛型信息,我们使用ofType获取
-->
<collection property="students" ofType="Student">
<result property="id" column="sid"/>
<result property="name" column="sname"/>
<result property="tid" column="tid"/>
</collection>
</resultMap>
<select id="getTeacher3" resultMap="TeacherStudent3">
select * from teacher where id = #{tid}
</select>
<resultMap id="TeacherStudent3" type="Teacher">
<!--<result property="id" column="id"/>-->
<collection property="students" javaType="ArrayList" ofType="Student" select="getStudentByTeacherId" column="id"/>
</resultMap>
<select id="getStudentByTeacherId" resultType="Student">
select * from student where tid = #{tid};
</select>
</mapper>
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