i am using a m2m and a through table, and when i was trying to save, my error was: Cannot set values on a ManyToManyField which specifies an intermediary model

so, i've modified my code, so that when i save the form, to insert data into the 'through' table too.But now, i'm having another error. (i've bolded the lines where i think i am wrong)

i have in models.py:

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

def save_classroom(request):
   if request.method == 'POST':

        form = ClassroomForm(request.POST, request.FILES, user = request.user) 
     **classroom_instance = Classroom
       member_instance = Membership**
        if form.is_valid():
           new_obj = form.save(commit=False)
           new_obj.user = request.user 
           r = Relations.objects.filter(initiated_by = request.user)
           membership = Membership.objects.create(**classroom = classroom_instance, member = member_instance,date=datetime.datetime.now())**
           new_obj.save()
           form.save_m2m()
           return HttpResponseRedirect('/classroom/classroom_view/{{user}}/')    
   else:
           form = ClassroomForm(user = request.user)     
   return render_to_response('classroom/classroom_form.html', {
           'form': form,

           }, 
          context_instance=RequestContext(request))  

but i don't seem to initialise okay the classroom_instance and menber_instance.My error os: Cannot assign "": "Membership.classroom" must be a "Classroom" instance.

Thanks!