title: Django save a list of items in a m2m intermediary table
body: <p>I have a m2m field in a class, and a through table. I am not table to save the list of items from the through table.
  if i have a multi select form, like below, and i want to be able to save all the selected items, how should i do it??</p>
  
  <p>My model form looks like this:</p>
  
  <pre><code>class ClassroomForm(ModelForm):
      class Meta:
            model = Classroom
            fields = ['classname','members','private']
  
      def __init__(self, *args, **kwargs):
                  creator = kwargs.pop('user')
    super(ClassroomForm, self).__init__(*args, **kwargs)
    relations = Relations.objects.filter(initiated_by = creator)
    self.fields["members"].queryset = \
         User.objects.filter(pk__in=[r.follow.pk for r in relations])
  </code></pre>
  
  <p>and my save method like this:</p>
  
  <pre><code>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() 
  </code></pre>
  
  <p>How this can be done?
  Thanks!</p>