Django更新而不是插入新记录

Question

我对Django 1.3有一个奇怪的问题.当我尝试使用ActivityForm添加新的Activity时,即使我显式创建一个不是Id的新Activity对象,Activity也经常更新.

此外,当我的ActivityForm类的init具有显式trip_id参数时,UI将显示"选择有效选项.该选项不是可用选项之一".对于字段位置(即使显示的选择有效).但是,如果我将它设为可选参数并从kwargs中弹出它,我就不会看到这个问题.

有人可以看看下面的代码,看看我做错了什么？

forms.py

class DestinationMultipleChoiceField(ModelMultipleChoiceField):
    def label_from_instance(self, obj):
        return obj.city_name

class ActivityForm(forms.Form):
    description = forms.CharField(max_length=100, required=True)
    location = DestinationChoiceField(queryset = Visit.objects.none(), empty_label=None, required=True)

    def __init__(self, trip_id = None, *args, **kwargs):
        super(ActivityForm, self).__init__(*args, **kwargs)
        if trip_id is not None:
            self.fields['location'].queryset = Visit.objects.filter(trip=trip_id).all().select_related('destination')

    # A new Activity() is created if nothing is provided
    def save(self, activity = Activity()):
        if not self.is_valid():
            raise forms.ValidationError('ActivityForm was not validated first before trying to call save().')

        activity.visit = self.cleaned_data['location']
        activity.description = self.cleaned_data['description']
        activity.added_by = User.objects.get(pk=1)
        activity.save()

views.py

def add_activity(request, trip_id = None, template_name = 'edit-form.html'):
    if request.POST:
        form = ActivityForm(trip_id, request.POST)
        if form.is_valid():
            form.save()
            return HttpResponseRedirect(reverse('views.list_trip'))
    else:
        form = ActivityForm(trip_id)

    return render_to_response(template_name, {
        'page_title': 'Add',
        'form': form,
    }, context_instance=RequestContext(request))

Answer 1

看看这一行:

def save(self, activity = Activity()):

在Python中,默认参数被评估一次; 因此,当第一次save调用该方法时,Activity将创建一个new ,但后续调用将使用该方法 Activity.

换成save这样的东西:

def save(self, activity=None):
    if activity is None:
        activity = Activity()
    # Rest of the method

然后,Activity将在每次调用时创建一个new (如果没有提供一个参数).