MayBatis中的 one to many （多的一方维护关系）

：首先理解什么叫维护关系: 就是维护关系的一方需要在数据库表中建立外键！

      学生的表（外键sgid）                                                             班级的表

                          

这里是学生（多的一方） 维护着关系  

    %%%%%%%%%%%%%%%%%%一的一方包含多的一方resultMap——list的写法 %%%%%%%%%%%%%%%%%%%%%%%%%%%%

   　　 <collection property="students"  ofType="com.ipcopyright.domain.Student">

      　　 　　 <id property="id" column="s_id"/>

        　　　　<result property="name" column="s_name"/>

    　　</collection>

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

班级Bean

public class Grade {

    private Integer gid;
    private String gname;
    private List<Students> studentsList = new ArrayList<Students>();

班级的映射

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >

<mapper namespace="com.cn.entity3.Grade" >
    <resultMap id="grad" type="com.cn.entity3.Grade">
        <id property="gid" column="gid"></id>
        <result property="gname" column="gname"></result>

        <collection property="studentsList" ofType="com.cn.entity3.Students">
            <id property="sid" column="sid"></id>
            <result property="sname" column="sname"></result>
        </collection>
    </resultMap>

　　　根据学生姓名查询所在班级
    <select id="ggg" resultMap="grad" parameterType="string">

        select g.gname,s.sname
        from grades g inner join students s
        on g.gid = s.sgid and s.sname = #{name}
    </select>
</mapper>

学生Bean

public class Students {
    private Integer sid;
    private String sname;
    private Grade grade;

学生的映射

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >

<mapper namespace="com.cn.entity3.Students" >
    <resultMap id="students" type="com.cn.entity3.Students">
        <id property="sid" column="sid"></id>
        <result property="sname" column="sname"></result>
        <!---->
        <association property="grade" resultMap="com.cn.entity3.Grade.grad"></association>
    </resultMap>

     
    //根据学生的名字查班级
    <select id="find" resultMap="students" parameterType="string">
        select s.sid,s.sname,g.gname
        from students s inner join grades g
        on s.sgid = g.gid where sname = #{sname}
    </select>
</mapper>