ValidReferenceConstraint.php

<?php

namespace Drupal\Core\Entity\Plugin\Validation\Constraint;

use Drupal\Core\StringTranslation\TranslatableMarkup;
use Drupal\Core\Validation\Attribute\Constraint;
use Symfony\Component\Validator\Constraint as SymfonyConstraint;

/**
 * Entity Reference valid reference constraint.
 *
 * Verifies that referenced entities are valid.
 */
class ValidReferenceConstraint extends SymfonyConstraint {
    
    /**
     * The default violation message.
     *
     * @var string
     */
    public $message = 'This entity (%type: %id) cannot be referenced.';
    
    /**
     * Violation message when the entity does not exist.
     *
     * @var string
     */
    public $nonExistingMessage = 'The referenced entity (%type: %id) does not exist.';
    
    /**
     * Violation message when a new entity ("autocreate") is invalid.
     *
     * @var string
     */
    public $invalidAutocreateMessage = 'This entity (%type: %label) cannot be referenced.';
    
    /**
     * Violation message when the target_id is empty.
     *
     * @var string
     */
    public $nullMessage = 'This value should not be null.';

}