Menu.php

<?php

namespace Drupal\system\Plugin\migrate\source;

use Drupal\migrate_drupal\Plugin\migrate\source\DrupalSqlBase;

/**
 * Menu source from database.
 *
 * @MigrateSource(
 *   id = "menu",
 *   source_module = "menu"
 * )
 */
class Menu extends DrupalSqlBase {
    
    /**
     * {@inheritdoc}
     */
    public function query() {
        return $this->select('menu_custom', 'm')
            ->fields('m');
    }
    
    /**
     * {@inheritdoc}
     */
    public function fields() {
        return [
            'menu_name' => $this->t('The menu name. Primary key.'),
            'title' => $this->t('The human-readable name of the menu.'),
            'description' => $this->t('A description of the menu'),
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function getIds() {
        $ids['menu_name']['type'] = 'string';
        return $ids;
    }

}