title: Update only specific fields in a models.Model
body: <p>I have a model </p>
  
  <pre><code>class Survey(models.Model):
      created_by = models.ForeignKey(User)
      question = models.CharField(max_length=150)
      active = models.NullBooleanField()
      def __unicode__(self):
          return self.question
  </code></pre>
  
  <p>and now I want to update only the <code>active</code> field. So I do this:</p>
  
  <pre><code>survey = get_object_or_404(Survey, created_by=request.user, pk=question_id)
  survey.active = True
  survey.save(["active"]) 
  </code></pre>
  
  <p>Now I get an error <code>IntegrityError: PRIMARY KEY must be unique</code>.</p>
  
  <p>Am I right with this method to update?</p>