title: How to save a django model with a manyToMany Through relationship, AND regular manyToMany relationships
body: <p>I've read everything I can find on ManyToMany relationships in Django, but so far I'm not seeing anything that solves my specific problem of having a ManyToMany-Through relationship alongside a simple ManyToMany that Django handles so effortlessly on its own.</p>
  
  <p>Consider these models:</p>
  
  <pre><code>class Treatment(models.Model):
      ...
      book = models.ManyToManyField(Book, through='TreatmentLocation')
      category = models.ManyToManyField(Category)
  
  class Book(models.Model):
      name = models.CharField(...)
  
  class TreatmentLocation(models.Model):
      treatment = models.ForeignKey(Treatment)
      book = models.ForeignKey(Book)
      page = models.CharField(...)
  
  class Category(models.Model):
      name = models.CharField(...)
  </code></pre>
  
  <p>I've got the data coming in on the POST array nicely, but finagling the View is proving tricky.</p>
  
  <pre><code>def save_treatment(request):
      form = TreatmentForm(request.POST)
  
      if form.is_valid():
          treatment = form.save()
  
          pages = request.POST.getlist('page')
          books = request.POST.getlist('book')
  
          counter = 0
          for page in pages:
              book_id = books.__getitem__(counter)
              TreatmentLocation.objects.create(treatment_id=treatment.id, book_id=book_id, page=page)
              counter = counter + 1
  
          form.save_m2m()
      else:
          ...
  </code></pre>
  
  <p>The Treatment saves successfully, as do the TreatmentLocations, but once I try to call save_m2m() to store the Treatment-Category xrefs, I get the <code>Cannot set values on a ManyToManyField which specifies an intermediary model. Use TreatmentLocation Manager</code> error.</p>
  
  <p>Does anyone know how to save both of these things? I'd like to avoid resorting to raw SQL.</p>