title: Update model fields based on POST data before save with Django Rest Framework
body: <p>I'm using django-rest-framework and want to augment the posted data before saving it to my model as is normally achieved using the model's clean method as in this example from the django docs:</p>
  
  <pre><code>class Article(models.Model):
  ...
  def clean(self):
      # Don't allow draft entries to have a pub_date.
      if self.status == 'draft' and self.pub_date is not None:
          raise ValidationError(_('Draft entries may not have a publication date.'))
      # Set the pub_date for published items if it hasn't been set already.
      if self.status == 'published' and self.pub_date is None:
          self.pub_date = datetime.date.today()
  </code></pre>
  
  <p>Unfortunately a django-rest-framework Serializer does not call a model's clean method as with a standard django Form so how would I achieve this?</p>