title: Django m2m form save " through " table
body: <p>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.</p>
  
  <p>my code:</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(User)
        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>and in the view:</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 should I initialise the membership for the Membership table to be populated right?</p>