expected one result (or null) to be returned by selectone(), but found: 5

一对多的情况下，需要确保主表有 id 标签，用于标识，让 mybatis 能够正常映射结果。

<resultMap id="rs1" type="Book" autoMapping="true">
    add:[<id property="id" column="id"/>]
    <collection property="tags" ofType="Tag" columnPrefix="bt_" autoMapping="true"/>
    <collection property="authors" ofType="Author" columnPrefix="ba_" autoMapping="true"/>
    <collection property="comments" ofType="Comment" columnPrefix="bc_" autoMapping="true"/>
    <collection property="previews" ofType="Preview" columnPrefix="bp_" autoMapping="true"/>
</resultMap>