TestValidatedReferenceConstraintValidator.php

<?php

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 $constraint) {
        if (!isset($items)) {
            return;
        }
        foreach ($items as $item) {
            $violations = $item->entity
                ->validate();
            foreach ($violations as $violation) {
                // Add the reason for the validation failure to the current context.
                $this->context
                    ->buildViolation($constraint->message)
                    ->addViolation();
            }
        }
    }

}