title: django Cannot set values on a ManyToManyField which specifies an intermediary model. Use Manager instead
body: <p>i am working on saving on the same form two tables - having a m2m relation.
  I don't succeed, my error persists with something like: Cannot set values on a ManyToManyField which specifies an intermediary model.  Use Membership's Manager instead
  where Membership is my 'through table'.</p>
  
  <p>my code : </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 = request.user,classroom=new_obj)
             membership.save() 
             form.save_m2m()
             return HttpResponseRedirect('.')    
     else:
             form = ClassroomForm(user = request.user)     
     return render_to_response('classroom/classroom_form.html', {
             'form': form,
  
             }, 
            context_instance=RequestContext(request))  
  </code></pre>
  
  <p>my models:</p>
  
  <pre><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(default=False)
        date = models.DateTimeField(auto_now = True) 
        classroom = models.ForeignKey(Classroom, related_name = 'classroom_membership')
        member = models.ForeignKey(User, related_name = 'user_membership')
  </code></pre>
  
  <p>where am i wrong?</p>