Sorting a customSQL column in smartGWT

Question

 <field name="application_id" foreignKey="Application.application_id" displayField="ref_number" keyField=application_id">
            
        </field>

<field name="ref_number" length="35" type="text" customSQL="true" foreignKey="Application.application_id">

        </field>

<binding operationType="fetch" operationId="fetchAppl">
            <selectClause>$defaultSelectClause<selectClause>
           <whereClause> $defaultWhereClause</whereClause>

        </binding>

I am trying to sort with smartgwt 6.1, but I noticed that when I try to sort with the ref_number which is a customSQL (not a column in this ds.xml file), I am getting error where no data is found. When I check the sql generated by smartGWT 6.1, I can see that the ref_number is not included in the query, I have tried to use orderClause, but it is not working as well. How can I do so