Add this

// redefine this in your model for additional fields
var $additionalFields = array();

function hasField($name) {
// handle fields
if(parent::hasField($name)) {
return true;
}
// maybe this model has a field that is added by a custom query
return in_array($name, $this->additionalFields);
}

to AppModel

and
var $additionalFields = array(‘count_stn’, ‘sum_stn’);
to your Models

This allowes you to use additional fields in your models by just adding

As far as I can see it from here, no modifications in the views are needed for asc / desc.

function hasField($fieldName){
     if(parent::hasField($fieldName))
          return true;
     else
          switch($fieldName){
               case "lowestprice":
               case "Custom.field_name1":
               case "Custom.field_name2":
               case "Custom.field_name3":
                    return true;
               default:
                    return false;
          }
     return false; // for good measure
}