如何在Django中清理动态表单字段

Question

我根据初始用户选择动态生成表单。问题是：如何验证动态生成的字段？到目前为止，这是我的代码：

#forms.py
class SelectionForm(forms.Form):
    what_to_order = forms.ChoiceField(
                      label=_('What do you want to order?'),
                      widget=forms.Select,
                      choices=[['invalid', 'Please Choose']] + sorted(choices.SELECTION_CHOICES))

    def __init__(self, *args, **kwargs):
        super(SelectionForm, self).__init__(*args, **kwargs)

        self.fields['what_to_order'].choices = \
            list(self.fields['what_to_order'].choices) + [('other', 'Other')]

    def clean_what_to_order(self):
        if self.cleaned_data['what_to_order'] == 'invalid':
            raise forms.ValidationError('Please Make a Choice!')

        return self.cleaned_data['what_to_order']

class OrderForm(forms.Form):
    name = forms.CharField(label='Name')

    def __init__(self, user_choice, *args, **kwargs):
        super(MyForm, self).__init__(*args, **kwargs)

        # CHOICES is a dict of choices and their fields' detail
        if user_choice in choices.CHOICES:
            for field in choices.CHOICES[user_choice]:
                self.fields[field] = form.CharField(
                                 label=choices.CHOICES[user_choice][field]['label'],
                                 help_text=choices.CHOICES[user_choice][field]['help_txt']

    def clean_name(self):
        if not self.cleaned_data['name']:
            raise forms.ValidationError('Missing')
        return self.cleaned_data['name']

    def clean_category(self):
        if not self.cleaned_data['category']:
            raise forms.ValidationError('Missing')
        return self.cleaned_data['category']

#views.py
def order(request):
    if request.method == 'POST':
        form = SelectionForm(request.POST)

        if form.is_valid():
            user_choice = form.cleaned_data['what_to_order']

            # this will use - make_order() - below
            return HttpResponseRedirect('/new_order/%s/' % (user_choice))

    else:
        form = SelectionForm()

    return render_to_response('app_name/new_order.html', { 'form': form })

def make_order(request, user_choice):
    if request.method == 'POST':
        form = OrderForm(None, request.POST)

        if form.is_valid():
            form.process()

            return HttpResponseRedirect('/new_order/thanks/')
    else:
        form = OrderForm(user_choice)

    return render_to_response('app_name/last_step_order.html', { 'form': form })

当OrderForm无效时，仅返回“name”字段。其他字段（基于用户输入生成的字段）从表单中消失。我不明白是什么导致它。