/**
  * @param DocumentInterface $document
  * @param Validator         $validator
  *
  * @return array
  */
 public function getValidationRules(DocumentInterface $document, Validator $validator)
 {
     $rules = parent::getValidationRules($document, $validator);
     if ($document->exists && ($key = array_search('required', $rules)) !== false) {
         $value = $document->getAttribute($this->getDBKey());
         if (!empty($value)) {
             unset($rules[$key]);
         }
     }
     return $rules;
 }