UniqueFieldConstraint.php

<?php

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

use Symfony\Component\Validator\Constraint;

/**
 * Checks if an entity field has a unique value.
 *
 * @Constraint(
 *   id = "UniqueField",
 *   label = @Translation("Unique field constraint", context = "Validation"),
 * )
 */
class UniqueFieldConstraint extends Constraint {
    public $message = 'A @entity_type with @field_name %value already exists.';
    
    /**
     * {@inheritdoc}
     */
    public function validatedBy() {
        return '\\Drupal\\Core\\Validation\\Plugin\\Validation\\Constraint\\UniqueFieldValueValidator';
    }

}