BookStack\Repos\PageRepo::changeBook PHP Method

    public function changeBook($bookId, Page $page)
    {
        $page->book_id = $bookId;
        foreach ($page->activity as $activity) {
            $activity->book_id = $bookId;
            $activity->save();
        }
        $page->slug = $this->findSuitableSlug($page->name, $bookId, $page->id);
        $page->save();
        return $page;
    }