/**
  * A usort callback to sort by methods on records
  * 
  * @param fActiveRecord $a  The first record to compare
  * @param fActiveRecord $b  The second record to compare
  * @return integer  < 0 if `$a` is less than `$b`, 0 if `$a` = `$b`, > 0 if `$a` is greater than `$b`
  */
 private function sortCallback($a, $b)
 {
     if ($this->sort_direction == 'asc') {
         return fUTF8::inatcmp($a->{$this->sort_method}(), $b->{$this->sort_method}());
     }
     return fUTF8::inatcmp($b->{$this->sort_method}(), $a->{$this->sort_method}());
 }