如何映射父id

Given this XML payload:

鉴于这种XML有效载荷:

<payload>
        <year yearNum="2013">
                <month monthNum="6" desc="This month was an enlightening month"/>
                <month monthNum="5" desc="This month was a questioning month"/>
                <month monthNum="4" desc="This month was a good month"/>
                <month monthNum="3" desc="This month was a crazy month"/>
                <month monthNum="2" desc="This month was a dry month"/>
                <month monthNum="1" desc="This month was a slow month"/>
        </year>
        <year yearNum="2012">
                <month monthNum="12" desc="This month was a cold month"/>
                <month monthNum="11" desc="This month was an expensive month"/>
                <month monthNum="10" desc="This month was a free month"/>
                <month monthNum="9" desc="This month was a hard month"/>
                <month monthNum="8" desc="This month was a surprising month"/>
                <month monthNum="7" desc="This month was an energetic month"/>
                <month monthNum="6" desc="This month was a hasty month"/>
                <month monthNum="5" desc="This month was a relaxing month"/>
                <month monthNum="4" desc="This month was a fair month"/>
                <month monthNum="3" desc="This month was a strange month"/>
                <month monthNum="2" desc="This month was a lucky month"/>
                <month monthNum="1" desc="This month was a odd month"/>
        </year>
</payload>
<paylo