[Zope] How do I sort on a dynamicly generated field?

I'm trying to get something like this going. <!--#in qryAQuery sort=doExternalMethod--> <tr> <td><!--#var doExternalMethod--></td> <td><!--#var AnotherField--></td> </tr> <!--#/in--> I'm sure I read somewhere you can sort on more than just the fields available in a table, ie calculated data etc but I can't seem to be able to do it. Any ideas?