Menu.php

<?php

namespace Drupal\system\Plugin\migrate\source;

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

/**
 * Drupal 6/7 menu source from database.
 *
 * For available configuration keys, refer to the parent classes.
 *
 * @see \Drupal\migrate\Plugin\migrate\source\SqlBase
 * @see \Drupal\migrate\Plugin\migrate\source\SourcePluginBase
 *
 * @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() {
        $fields = [
            'menu_name' => $this->t('The menu name. Primary key.'),
            'title' => $this->t('The human-readable name of the menu.'),
            'description' => $this->t('A description of the menu'),
        ];
        // The database connection may not exist, for example, when building
        // the Migrate Message form.
        if ($source_database = $this->database) {
            if ($source_database->schema()
                ->fieldExists('menu_custom', 'language')) {
                $fields += [
                    'language' => $this->t('Menu language.'),
                    'i8n_mode' => $this->t('Menu i18n mode.'),
                ];
            }
        }
        return $fields;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getIds() {
        $ids['menu_name']['type'] = 'string';
        return $ids;
    }

}