protected function _beforeSave(Kwf_Model_Row_Interface $row, $submitRow)
 {
     parent::_beforeSave($row, $submitRow);
     foreach ($this->_colNames as $colName) {
         if (!$row->{$colName}) {
             $row->{$colName} = null;
         } else {
             if ($row->{$colName} == ' ') {
                 $row->{$colName} = '';
             }
         }
     }
 }