Question

I having a weird issue with Django 1.3. When I try to add a new Activity using the ActivityForm, the Activity is often updated even if I explicitly create a new Activity object with not Id.

In addition, when the init of my ActivityForm class has an explicit trip_id parameter, the UI displays “Select a valid choice. That choice is not one of the available choices.” for the field location (even though the selection shown is valid). However, if I make this an optional parameter and I pop it from kwargs, I don’t see this issue.

Can someone please take a look at the code below and see what I am doing wrong?

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

Look at this line:

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

In Python, default parameters are evaluated once; thus, when the save method is called the first time, a new Activity will be created, but subsequent calls will use that Activity.

Change save to something like this:

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

Then, a new Activity will be created on every call (if one is not supplied as an argument).