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 custom block/box content.'),
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function getIds() {
        $ids['bid']['type'] = 'integer';
        return $ids;
    }

}