Config.php

<?php

namespace Drupal\migrate_drupal\Plugin\migrate\source\d8;

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

/**
 * Drupal config source from database.
 *
 * @MigrateSource(
 *   id = "d8_config",
 *   source_module = "system",
 * )
 */
class Config extends DrupalSqlBase {
    
    /**
     * {@inheritdoc}
     */
    public function query() {
        $query = $this->select('config', 'c')
            ->fields('c', [
            'collection',
            'name',
            'data',
        ]);
        if (!empty($this->configuration['collections'])) {
            $query->condition('collection', (array) $this->configuration['collections'], 'IN');
        }
        if (!empty($this->configuration['names'])) {
            $query->condition('name', (array) $this->configuration['names'], 'IN');
        }
        return $query;
    }
    
    /**
     * {@inheritdoc}
     */
    public function prepareRow(Row $row) {
        $row->setSourceProperty('data', unserialize($row->getSourceProperty('data')));
    }
    
    /**
     * {@inheritdoc}
     */
    public function fields() {
        return [
            'collection' => $this->t('The config object collection.'),
            'name' => $this->t('The config object name.'),
            'data' => $this->t('Serialized configuration object data.'),
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function getIds() {
        $ids['collection']['type'] = 'string';
        $ids['name']['type'] = 'string';
        return $ids;
    }

}