Box.php

<?php

namespace Drupal\block_content\Plugin\migrate\source\d6;

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

/**
 * Drupal 6 block 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 = "d6_box",
 *   source_module = "block"
 * )
 */
class Box extends DrupalSqlBase {
  
  /**
   * {@inheritdoc}
   */
  public function query() {
    $query = $this->select('boxes', 'b')
      ->fields('b', [
      'bid',
      'body',
      'info',
      'format',
    ]);
    $query->orderBy('b.bid');
    return $query;
  }
  
  /**
   * {@inheritdoc}
   */
  public function fields() {
    return [
      'bid' => $this->t('The numeric identifier of the block/box'),
      'body' => $this->t('The block/box content'),
      'info' => $this->t('Admin title of the block/box.'),
      'format' => $this->t('Input format of the content block/box content.'),
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function getIds() {
    $ids['bid']['type'] = 'integer';
    return $ids;
  }

}