What are the steps to make a ModelForm work with a ManyToMany relationship with an intermediary model in Django?

Question

• I have a Client and Groupe Model.
• A Client can be part of multiple groups.
• Clients that are part of a gr

Answer 1

If you use the save method right now, Django will try to save using the manager (which Django doesn't allow). Unfortunately, the behavior you want is a little bit trickier than what ModelForm does by default. What you need to do is create a formset.

First of all, you will need to change the options of your ClientForm so that it doesn't display the groupes attribute.

class ClientForm(ModelForm):
    class Meta:
        model = Client
        exclude = ('groupes',)

Next, you must change the view to display the formset:

from django.forms.models import inlineformset_factory

def modifier(request, id):
    client = Client.objects.get(id=id)    
    form = ClientForm(instance = client)
    # Create the formset class
    GroupeFormset = inlineformset_factory(Client, Groupe)
    # Create the formset
    formset = GroupeFormset(instance = client)

    dict = {
        "form": form
        , "formset" : formset
        , "instance" : client
    }

    if request.method == "POST":
        form = ClientForm(request.POST, instance = client)
        formset = GroupeFormset(request.POST, instance = client)

        if form.is_valid() and formset.is_valid():
            client_mod = form.save()
            formset.save()

            id = client_mod.id
            return HttpResponseRedirect(
                "/client/%(id)s/?err=success" % {"id" : id}
            )
        else:
            return HttpResponseRedirect(
                "/client/%(id)s/?err=warning" % {"id" : id}
            )

    return render_to_response(
        "client/modifier.html"
        , dict
        , context_instance=RequestContext(request)
    )

And obviously, you must also tweak your template to render the formset.

If you need any other advice on formsets, see these articles:

Model formsets
Formsets