How to use DIH to index attributes in xml file

[Jianbin Dai]

I have an xml file like this 

<merchantProduct id="814636051" mid="189973">
                    <in_stock type="stock-4" />
                    <condition type="cond-0" />
                    <price>301.46</price>
</merchantProduct>

In the data-config.xml, I use
<field column="price"    xpath="/.../merchantProduct/price" />

but how can I index "id", "mid"?

Thanks.