Django视图内处理ManyToMany表单字段创建保存新模型实例失败如何解决

错误原因 #

Django的多对多字段对应的数据存储在独立的中间关联表中，需要先保存主模型实例（这里是Pollers）获得主键ID后，才能操作中间表完成关联。直接在实例化主模型时给多对多字段赋值是不被允许的，这也是报错提示的核心含义。

代码调整步骤 #

只需要修改views.py中保存实例的逻辑即可：

1. 实例化Pollers时移除poller_categories的直接赋值
2. 主实例保存完成后，调用多对多字段的set()方法传入选中的分类列表

修改后的views.py代码 #

def raise_poller(request):
        # if this is a POST request we need to process the form data
        if request.method == 'POST':

            # create a form instance and populate it with data from the request:
            form = PollersForm(request.POST)

            # check whether it's valid:
            if form.is_valid():
                # process the data in form.cleaned_data as required

                poller_nature = form.cleaned_data['poller_nature']
                poller_text = form.cleaned_data['poller_text']
                poller_choice_one = form.cleaned_data['poller_choice_one']
                poller_choice_two = form.cleaned_data['poller_choice_two']
                poller_categories = form.cleaned_data['poller_categories']

                # Get the user
                created_by = request.user

                # 保存Poller主实例，去掉多对多字段的直接赋值
                p = Pollers(poller_nature = poller_nature,
                            poller_text = poller_text,
                            poller_choice_one = poller_choice_one,
                            poller_choice_two = poller_choice_two,
                            created_by = created_by)
                p.save()
                # 主实例保存成功后，用set方法设置多对多关联
                p.poller_categories.set(poller_categories)

                # redirect to a new URL:
                return HttpResponseRedirect('/')

补充说明 #

如果后续需要给多对多字段追加关联对象，也可以用add()方法，比如p.poller_categories.add(category_instance)，清空所有关联可以用clear()方法。

内容的提问来源于stack exchange，提问作者JSRB