OrderBy

   Is it possible to OrderBy a spanned column in an RdbQuery?  What I would like to do is this:

            // Query
            _query = new RdbQuery(typeof(AS400Client));

            // Where clause
            _query.AddClause(    AS400Client.FirstNameEntityColumn, 
                                EntityQueryOp.EQ, 
                                "David");

            // Add span to other table
            _query.AddSpan(EntityRelations.AS400Workplace_AS400Client);

            // Add orderby on column in *related* table
            EntityOrderBy orderBy = new EntityOrderBy(AS400Workplace.DescriptionEntityColumn, ListSortDirection.Descending);

            _query.ClearOrderBy();            
            _query.AddOrderBy(orderBy);

            // Get the clients 
            PersistenceManager pm = PersistenceManager.DefaultManager;
            _clients = pm.GetEntities<AS400Client>(_query);   

Of course, I get an error:  "Cannot find column Description."   Is there any way of doing this?

(The reason why I'm not simply defining the query as being of the AS400Workplace type is that I want, potentially, to order by columns in either table, dynamically, depending on user input).