Book.php

<?php

namespace Drupal\book\Plugin\migrate\source;

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

/**
 * Drupal 6/7 book 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 = "book",
 *   source_module = "book",
 * )
 */
class Book extends DrupalSqlBase {
    
    /**
     * {@inheritdoc}
     */
    public function query() {
        $query = $this->select('book', 'b')
            ->fields('b', [
            'nid',
            'bid',
        ]);
        $query->join('menu_links', 'ml', '[b].[mlid] = [ml].[mlid]');
        $ml_fields = [
            'mlid',
            'plid',
            'weight',
            'has_children',
            'depth',
        ];
        foreach (range(1, 9) as $i) {
            $field = "p{$i}";
            $ml_fields[] = $field;
            $query->orderBy('ml.' . $field);
        }
        return $query->fields('ml', $ml_fields);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getIds() {
        $ids['mlid']['type'] = 'integer';
        $ids['mlid']['alias'] = 'ml';
        return $ids;
    }
    
    /**
     * {@inheritdoc}
     */
    public function fields() {
        return [
            'nid' => $this->t('Node ID'),
            'bid' => $this->t('Book ID'),
            'mlid' => $this->t('Menu link ID'),
            'plid' => $this->t('Parent link ID'),
            'weight' => $this->t('Weight'),
            'p1' => $this->t('The first mlid in the materialized path. If N = depth, then pN must equal the mlid. If depth > 1 then p(N-1) must equal the parent link mlid. All pX where X > depth must equal zero. The columns p1 .. p9 are also called the parents.'),
            'p2' => $this->t('The second mlid in the materialized path. See p1.'),
            'p3' => $this->t('The third mlid in the materialized path. See p1.'),
            'p4' => $this->t('The fourth mlid in the materialized path. See p1.'),
            'p5' => $this->t('The fifth mlid in the materialized path. See p1.'),
            'p6' => $this->t('The sixth mlid in the materialized path. See p1.'),
            'p7' => $this->t('The seventh mlid in the materialized path. See p1.'),
            'p8' => $this->t('The eighth mlid in the materialized path. See p1.'),
            'p9' => $this->t('The ninth mlid in the materialized path. See p1.'),
        ];
    }

}