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

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

You probably need to remove the ManyToMany field from your Client model, or else carefully exclude it from your form. Unfortunately, the default widget for the ManyToMany field cannot populate the ClientGroupe Model properly (even if the missing field, dt, had been set to autonow=True). This is something you’ll either need to break out into another form, or handle in your view.

When you save your form, you save Client object. Now if you want to assign client to the group you should do this:

clientgroupe = ClientGroupe.objects.create(client=client_instance, groupe=groupe_instance, dt=datetime.datetime.now())

where client_instance and groupe_instance your client and groupe objets.

I’m providing an alternative solution due to issues I encountered with forms_valid not being called:

class SplingCreate(forms.ModelForm):
class Meta:
    model = SplingModel
    fields = ('Link', 'Genres', 'Image', 'ImageURL',)

def save(self, commit=True):
    from django.forms.models import save_instance

    if self.instance.pk is None:
        fail_message = 'created'
    else:
        fail_message = 'changed'
    fields = set(self._meta.fields) - set(('Genres',))
    instance = save_instance(self, self.instance, fields,
                             fail_message, commit, construct=False)

    genres = self.cleaned_data.get('Genres')
    for genre in genres:
        SplingGenreModel.objects.get_or_create(spling=instance, genre=genre)

    return instance

I’ve copied the logic from djangos forms/models.py, my field Genres is a manytomany with an intermediary table – I exclude it from the save_instance and then save it separately.