解决Mybatis 一对多分布查询中集合内对象为空

<resultMap id="deplist" type="DepList">
        <id property="dId" column="d_id"></id>
        <result property="dName" column="d_name"></result>
        <collection property="list" select="mapper.empMapper.selectById" column="d_id">
        </collection>
 6 </resultMap>
<select id="selectById" resultMap="deplist">
        select *
        from dep
        where d_id = #{id}
11 </select>    

    <resultMap id="em" type="emp">  <!-- 缺少这部分映射关系会导致结果list中的元素都为 null -->
        <id property="eId" column="e_id"></id>
        <result property="eName" column="e_name"></result>
        <result property="dId" column="e_d"></result>
    </resultMap>
    <select id="selectById" resultMap="em">
        select * from emp where e_d = #{id}
    </select>