unique_field_constraint_test.module

<?php


/**
 * @file
 * Contains unique_field_constraint_test.module.
 */
use Drupal\Core\Entity\EntityTypeInterface;

/**
 * Implements hook_entity_base_field_info_alter().
 */
function unique_field_constraint_test_entity_base_field_info_alter(&$fields, EntityTypeInterface $entity_type) {
    if ($entity_type->id() === 'entity_test_string_id') {
        
        /** @var \Drupal\Core\Field\BaseFieldDefinition[] $fields */
        $fields['name']->addConstraint('UniqueField');
    }
}