TestValidatedReferenceConstraintValidator.php

<?php

declare (strict_types=1);
namespace Drupal\entity_test\Plugin\Validation\Constraint;

use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;

/**
 * Validates referenced entities.
 */
class TestValidatedReferenceConstraintValidator extends ConstraintValidator {
  
  /**
   * {@inheritdoc}
   */
  public function validate($items, Constraint $constraint) : void {
    if (!isset($items)) {
      return;
    }
    foreach ($items as $item) {
      $violations = $item->entity
        ->validate();
      if ($violations->count()) {
        // Add the reason for the validation failure to the current context.
        $this->context
          ->buildViolation($constraint->message)
          ->addViolation();
      }
    }
  }

}