django forms: editing multiple sets of related objects in a single form

In the end I opted for processing multiple forms within the same view, a Visitor model form for the visitor details, then a list of custom forms for each of the destinations.

Processing multiple forms in the same view turned out to be simple enough (at least in this case, where there were no cross-field validation issues).

I'm still surprised there is no built-in support for many to many relationships with an intermediary model, and looking around in the web I found no direct reference to it. I'll post the code in case it helps anyone.

First the custom forms:

class VisitorForm(ModelForm):
    class Meta:
      model = Visitor
      exclude = ['destinations']

class VisitorDestinationForm(Form):
    visited = forms.BooleanField(required=False)
    activities = forms.MultipleChoiceField(choices = [(obj.pk, obj.name) for obj in Activity.objects.all()], required=False, 
                                                      widget = CheckboxSelectMultipleInline(attrs={'style' : 'display:inline'}))

    def __init__(self, visitor, destination, visited,  *args, **kwargs):
        super(VisitorDestinationForm, self).__init__(*args, **kwargs)
        self.destination = destination
        self.fields['visited'].initial = visited
        self.fields['visited'].label= destination.destination

        # load initial choices for activities
        activities_initial = []
        try:
            visitorDestination_entry = VisitorDestination.objects.get(visitor=visitor, destination=destination)
            activities = visitorDestination_entry.activities.all()
            for activity in Activity.objects.all():
                if activity in activities: 
                    activities_initial.append(activity.pk)
        except VisitorDestination.DoesNotExist:
            pass
        self.fields['activities'].initial = activities_initial

I customize each form by passing a Visitor and Destination objects (and a 'visited' flag which is calculated outside for convenience)

I use a boolean field to allow the user to select each destination. The field is called 'visited', however I set the label to the destination so it gets nicely displayed.

The activities get handled by the usual MultipleChoiceField (I used I customized widget to get the checkboxes to display on a table, pretty simple but can post it if somebody needs that)

Then the view code:

def edit_visitor(request, pk):
    visitor_obj = Visitor.objects.get(pk=pk)
    visitorDestinations = visitor_obj.destinations.all()
    if request.method == 'POST':
        visitorForm = VisitorForm(request.POST, instance=visitor_obj)

        # set up the visitor destination forms
        destinationForms = []
        for destination in Destination.objects.all():
            visited = destination in visitorDestinations
            destinationForms.append(VisitorDestinationForm(visitor_obj, destination, visited, request.POST, prefix=destination.destination))

        if visitorForm.is_valid() and all([form.is_valid() for form in destinationForms]):
            visitor_obj = visitorForm.save()
            # clear any existing entries,
            visitor_obj.destinations.clear()
            for form in destinationForms:
                if form.cleaned_data['visited']: 
                    visitorDestination_entry = VisitorDestination(visitor = visitor_obj, destination=form.destination)
                    visitorDestination_entry.save()
                    for activity_pk in form.cleaned_data['activities']: 
                        activity = Activity.objects.get(pk=activity_pk)
                        visitorDestination_entry.activities.add(activity)
                    print 'activities: %s' % visitorDestination_entry.activities.all()
                    visitorDestination_entry.save()

            success_url = reverse('visitor_detail', kwargs={'pk' : visitor_obj.pk})
            return HttpResponseRedirect(success_url)
    else:
        visitorForm = VisitorForm(instance=visitor_obj)
        # set up the visitor destination forms
        destinationForms = []
        for destination in Destination.objects.all():
            visited = destination in visitorDestinations
            destinationForms.append(VisitorDestinationForm(visitor_obj, destination, visited,  prefix=destination.destination))

    return render_to_response('testapp/edit_visitor.html', {'form': visitorForm, 'destinationForms' : destinationForms, 'visitor' : visitor_obj}, context_instance= RequestContext(request))

I simply collect my destination forms in a list and pass this list to my template, so that it can iterate over them and display them. It works well as long as you don't forget to pass a different prefix for each one in the constructor

I'll leave the question open for a few days in case some one has a cleaner method.

Thanks!