/**
  * Validates a single attribute.
  *
  * @param CModel $object    the data object being validated
  * @param string $attribute the name of the attribute to be validated.
  */
 protected function validateAttribute($object, $attribute)
 {
     if (!$object->isAttributeDirty($attribute)) {
         return;
     }
     if ($this->model === null) {
         //currently unsupported
         return;
     }
     if ($this->message === null) {
         $this->message = '{attribute} can not be edited because it is already in use';
     }
     if ($this->foreignKey === null) {
         $this->foreignKey = $object->tableName() . '_id';
     }
     if ($this->validateContainedInModel($object->id)) {
         $this->addError($object, $attribute, $this->message);
     }
 }