title: Set value of field for Django ModelForm in CreateView
body: <p>I need to set the value of a BooleanField from my Django model via the CreateView for my ModelForm. But for some reason, setting it in form_valid() isn't working.</p>
  
  <p>Here's my model:</p>
  
  <pre><code>class Feedback(models.Model):
      was_satisifed = models.BooleanField(
          help_text='Returns true if the user exits the process early.',
          default=False)
  </code></pre>
  
  <p>Here's my view:</p>
  
  <pre><code>class FeedbackActionMixin(object):
      model = Feedback
      form_class = FeedbackForm
  
      def form_valid(self, form):
          instance = form.save(commit=False)
          instance.was_satisfied = True
          return super(FeedbackActionMixin, self).form_valid(form)
  </code></pre>
  
  <p>The form submits, but the "was_satisfied" value is left at the default False. What am I missing?</p>