Upload.php

<?php

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

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

/**
 * Drupal 6 upload 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_upload",
 *   source_module = "upload"
 * )
 */
class Upload extends DrupalSqlBase {
    
    /**
     * The join options between the node and the upload table.
     */
    const JOIN = '[n].[nid] = [u].[nid] AND [n].[vid] = [u].[vid]';
    
    /**
     * {@inheritdoc}
     */
    public function query() {
        $query = $this->select('upload', 'u')
            ->distinct()
            ->fields('u', [
            'nid',
            'vid',
        ]);
        $query->innerJoin('node', 'n', static::JOIN);
        $query->addField('n', 'type');
        $query->addField('n', 'language');
        return $query;
    }
    
    /**
     * {@inheritdoc}
     */
    public function prepareRow(Row $row) {
        $query = $this->select('upload', 'u')
            ->fields('u', [
            'fid',
            'description',
            'list',
        ])
            ->condition('u.nid', $row->getSourceProperty('nid'))
            ->orderBy('u.weight');
        $query->innerJoin('node', 'n', static::JOIN);
        $row->setSourceProperty('upload', $query->execute()
            ->fetchAll());
        return parent::prepareRow($row);
    }
    
    /**
     * {@inheritdoc}
     */
    public function fields() {
        return [
            'fid' => $this->t('The file Id.'),
            'nid' => $this->t('The node Id.'),
            'vid' => $this->t('The version Id.'),
            'type' => $this->t('The node type'),
            'language' => $this->t('The node language.'),
            'description' => $this->t('The file description.'),
            'list' => $this->t('Whether the list should be visible on the node page.'),
            'weight' => $this->t('The file weight.'),
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function getIds() {
        $ids['vid']['type'] = 'integer';
        $ids['vid']['alias'] = 'u';
        return $ids;
    }

}