django添加数据到库里，库里意见显示数据了，但是打印为

def add(request):
    if request.method == 'POST':
        title = request.POST.get("title")
        price = request.POST.get("price")
        pub_date = request.POST.get("pub_date")
        publish_id = request.POST.get("publish_id")
        authors_id_list = request.POST.getlist('authors_id_list')  # checkbox select
        book_obj = Book.objects.create(title=title, price=price, publishDate=pub_date, publish_id=publish_id)
        print(authors_id_list, book_obj)

        # book_obj.author.add(*authors_id_list)
        return HttpResponse('ok')

打印结果为：

            ['2']   Book object (None)

你的BOOK表的def __str__()方法怎么写的？？