Formset不会创建新记录，只会更新旧记录

Question

我在一个页面中有formset和form，两者都是同时保存的。但是不保存事件时间的新记录，只更新旧记录。问题在哪里？

表单集：

    class EventTimeForm(forms.ModelForm):
        class Meta:
            model = EventTime
            fields = ['start_time']
            widgets = {
                'start_time': DateTimeWidget(usel10n=True, bootstrap_version=3)
            }
EventTimeFormSet = modelformset_factory(EventTime, form=EventTimeForm, max_num=3, extra=3)

形式：

class CreateEventForm(forms.ModelForm):
    class Meta:
        model = Event
        fields = ['title', 'description', 'location', 'duaration', 'private']

观点：

def create_event(request):
    event_form = CreateEventForm(request.POST or None, prefix='event')
    event_time_form = EventTimeFormSet(request.POST or None, prefix='eventtime')
    context = {
        'event_form': event_form,
        'event_time_form': event_time_form,
    }
    if event_form.is_valid() and event_time_form.is_valid():
        event = event_form.save()
        event_time = event_time_form.save()
        for obj in event_time:
            obj.event.add(event)
        return HttpResponseRedirect('/event-%s' %event.id)
    return render(request, 'create_event.html', context)

模板：

<form method='POST' action=''>{%csrf_token%}
    {{ event_time_form|crispy }}
    {{ event_form|crispy}}
    <input class="btn btn-primary" type="submit" value="Create" />
</form>