private function fetchTargetRows()
    {
        $model  = $this->targetModel();
        $source = $model->source();

        $sql = strtr(
            'SELECT %key FROM `%table`',
            [
                '%table' => $source->table(),
                '%key'   => $this->oldPrimaryKey(),
            ]
        );

        return $source->db()->query($sql, PDO::FETCH_ASSOC);
    }