code: public function validate($value, Constraint $constraint)
      {
          $result = $this->repository->findOneBySiteId($value->getSiteId());
  
          if (null !== $result && $result->getId() !== $value->getId()) {
              $this->context->buildViolation($constraint->message)
                  ->atPath('siteId')
                  ->addViolation();
          }
      }
docs: Checks if the passed value is valid.
  
  @param SiteInterface              $value      The value that should be validated
  @param UniqueMainAlias|Constraint $constraint The constraint for the validation
  
  @api