How to sort by foreign table column

[xxshutong]

Hi Guys,
I have a list page with form as bellow:
<service service-name="performFind" result-map="result" 
result-map-list="listIt">
          <field-map field-name="inputFields" from-field="inputCtx"/>
          <field-map field-name="entityName" value="InventoryInput"/>
          <field-map field-name="orderBy" value="productId"/>
          <field-map field-name="viewIndex" from-field="viewIndex"/>
          <field-map field-name="viewSize" from-field="viewSize"/>
</service>

productId is a foreign key, how can I sort by a foreign column.
like <field-map field-name="orderBy" value="product.name"/> can not work.

Thanks,
Leon.Zhou