django forms: editing multiple sets of related obj

Question

I'm trying to do something that should be very common: add/edit a bunch of related models in a single form. For example:

Visitor Details:
Select destinations and activities:
    Miami  []   -  swimming [], clubbing [], sunbathing[]
    Cancun []   -  swimming [], clubbing [], sunbathing[]

My models are Visitor, Destination and Activity, with Visitor having a ManyToMany field into Destination through an intermediary model, VisitorDestination, which has the details of the activities to be done on the destination (in itself a ManyToMany field into Activity).

Visitor ---->(M2M though VisitorDestination) -------------> Destination
                                            |
                       activities            ---->(M2M)---> Activity  

Note that I don't want to enter new destination / activity values, just choose from those available in the db (but that's a perfectly legit use of M2M fields right?)

To me this looks like an extremely common situation (a many to many relation with additional details which are a FK or M2M field into some other model), and this looks like the most sensible modelling, but please correct me if I'm wrong.

I've spent a few days searching Django docs / SO / googling but haven't been able to work out how to deal with this. I tried several approaches:

1. Custom Model form for Visitor, where I add multiple choice fields for Destination and Activity. That works ok if Destination and Activity could be selected independently, but here they are correlated, ie I want to choose one or several activities for each destination

2. Using inlineformset_factory to generate the set of destination / activities forms, with inlineformset_factory(Destination, Visitor). This breaks, because Visitor has a M2M relation to Destination, rather than a FK.

3. Customizing a plain formset, using formset_factory, eg DestinationActivityFormSet = formset_factory(DestinationActivityForm, extra=2). But how to design DestinationActivityForm? I haven't explored this enough, but it doesn't look very promising: I don't want to type in the destination and a list of activities, I want a list of checkboxes with the labels set to the destination / activities I want to select, but the formset_factory would return a list of forms with identical labels.

I'm a complete newbie with django so maybe the solution is obvious, but I find that the documentation in this area is very weak - if anyone has some pointers to examples of use for forms / formsets that would be also helpful

thanks!

Answer 1

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!

Answer 2

So, as you've seen, one of the things about inlineformset_factory is that it expects two models - a parent, and child, which has a foreign key relationship to the parent. How do you pass extra data on the fly to the form, for extra data in the intermediary model?

How I do this is by using curry:

from django.utils.functional import curry

from my_app.models import ParentModel, ChildModel, SomeOtherModel

def some_view(request, child_id, extra_object_id):
    instance = ChildModel.objects.get(pk=child_id)
    some_extra_model = SomeOtherModel.objects.get(pk=extra_object_id)

    MyFormset = inlineformset_factory(ParentModel, ChildModel, form=ChildModelForm)

    #This is where the object "some_extra_model" gets passed to each form via the
    #static method
    MyFormset.form = staticmethod(curry(ChildModelForm,
        some_extra_model=some_extra_model))

    formset = MyFormset(request.POST or None, request.FILES or None,
        queryset=SomeObject.objects.filter(something=something), instance=instance)

The form class "ChildModelForm" would need to have an init override that adds the "some_extra_model" object from the arguments:

def ChildModelForm(forms.ModelForm):
    class Meta:
        model = ChildModel

    def __init__(self, some_extra_model, *args, **kwargs):
        super(ChildModelForm, self).__init__(*args, **kwargs)
        #do something with "some_extra_model" here

Hope that helps get you on the right track.

Answer 3

From django 1.9, there is a support for passing custom parameters to formset forms : https://docs.djangoproject.com/en/1.9/topics/forms/formsets/#passing-custom-parameters-to-formset-forms

Just add form_kwargs to your FormSet init like this :

from my_app.models import ParentModel, ChildModel, SomeOtherModel

def some_view(request, child_id, extra_object_id):
    instance = ChildModel.objects.get(pk=child_id)
    some_extra_model = SomeOtherModel.objects.get(pk=extra_object_id)

    MyFormset = inlineformset_factory(ParentModel, ChildModel, form=ChildModelForm)
    formset = MyFormset(request.POST or None, request.FILES or None,
        queryset=SomeObject.objects.filter(something=something), instance=instance,
        form_kwargs={"some_extra_model": some_extra_model})