Django m2m form save " through " table

I'm having trouble in saving a m2m data, containing a 'through' class table. I want to save all selected members (selected in the form) in the through table. But i don't know how to initialise the 'through' table in the view.

my code:

class Classroom(models.Model):
     user = models.ForeignKey(User, related_name = 'classroom_creator')
     classname = models.CharField(max_length=140, unique = True)
     date = models.DateTimeField(auto_now=True)
     open_class = models.BooleanField(default=True)
     members = models.ManyToManyField(User,related_name="list of invited members", through = 'Membership')

class Membership(models.Model): 
      accept = models.BooleanField(User)
      date = models.DateTimeField(auto_now = True) 
      classroom = models.ForeignKey(Classroom, related_name = 'classroom_membership')
      member = models.ForeignKey(User, related_name = 'user_membership')

and in the view:

def save_classroom(request):
   classroom_instance = Classroom()
   if request.method == 'POST':
        form = ClassroomForm(request.POST, request.FILES, user = request.user) 
        if form.is_valid():
           new_obj = form.save(commit=False)
           new_obj.user = request.user 
           new_obj.save()
           membership = Membership(member = HERE SELECTED ITEMS FROM FORM,classroom=new_obj)

           membership.save() 

How should I initialise the membership for the Membership table to be populated right?

In case of using normal m2m relation (not through intermediary table) you could replace:

membership = Membership(member = HERE SELECTED ITEMS FROM FORM,classroom=new_obj)
membership.save()

with

form.save_m2m()

But in case of using intermediary tables you need to manually handle POST data and create Membership objects with all required fields (similar problem). The most basic solution is to change your view to something like:

def save_classroom(request):
    if request.method == 'POST':
        form = ClassroomForm(request.POST, request.FILES)

        if form.is_valid():
           new_obj = form.save(commit=False)
           new_obj.user = request.user 
           new_obj.save()

           for member_id in request.POST.getlist('members'):
                membership = Membership.objects.create(member_id = int(member_id), classroom = new_obj)
           return HttpResponseRedirect('/')
    else:
        form = ClassroomForm()
    return render_to_response('save_classroom.html', locals())

Note how request.POST is manipulated (.getlist). This is because post and get are QueryDict objects which has some implications (request.POST['members'] will return always one object!).

You can modify this code to get it more reliable (error handling etc.), and more verbose, eg:

member = get_object_or_404(User, pk = member_id)
membership = Membership.objects.create(member = member , classroom = new_obj)

But note that you are performing some db queries in a loop which is not a good idea in general (in terms of performance).

Like what dzida did, but use form.cleaned_data instead of request.post:

def save_classroom(request):
    if request.method == 'POST':
        form = ClassroomForm(request.POST, request.FILES)

        if form.is_valid():
           new_obj = form.save(commit=False)
           new_obj.user = request.user
           new_obj.save()

           for member in form.cleaned_data['members'].all():
                Membership.objects.create(member = member,  classroom = new_obj)

           return HttpResponseRedirect('/')
    else:
        form = ClassroomForm()
    return render_to_response('save_classroom.html', locals())

You also need to consider some memberships might be deleted, so:

def save_classroom(request):
    if request.method == 'POST':
        form = ClassroomForm(request.POST, request.FILES)

        if form.is_valid():
           new_obj = form.save(commit=False)
           new_obj.user = request.user
           new_obj.save()

           final_members = form.cleaned_data['members'].all()
           initial_members = form.initial['members'].all()

           # create and save new members
           for member in final_members:
                if member not in initial_members:
                    Membership.objects.create(member = member,  classroom = new_obj)

           # delete old members that were removed from the form
           for member in initial_members:
               if member not in final_members:
                   Membership.objects.filter(member = member, classroom = new_obj).delete()

           return HttpResponseRedirect('/')
    else:
        form = ClassroomForm()
    return render_to_response('save_classroom.html', locals())

If you use model forms (like in a generic CBV: form_class=ClassroomForm), override and put the saving logic above in the save method, something like:

ClassroomForm(forms.ModelForm):
    members = ModelMultipleChoiceField(
        queryset=Classroom.objects.all(),
        widget=SelectMultiple
    )

    def save(self, commit=True):
        classroom = super().save(commit=False)
            if commit:
                classroom.save()
                if 'members' in self.changed_data:
                   final_members = self.cleaned_data['members'].all()
                   initial_members = self.initial['members']

                   # create and save new members
                   for member in final_members:
                        if member not in initial_members:
                            Membership.objects.create(member = member,  classroom = new_obj)

                   # delete old members that were removed from the form
                   for member in initial_members:
                       if member not in final_members:
                           Membership.objects.filter(member = member, classroom = new_obj).delete()

         return classroom