title: save() method for django_comments_xtd model
body: <p>I am using Wagtail + Django_comments_xtd + Django.</p>
  <h4>My_Django_App/models.py</h4>
  <pre><code>from wagtail.core.models import Page
  class PostPage(Page):
    ...
  
  
  from django_comments_xtd.models import XtdComment
  class PostComment(XtdComment):
      page = ParentalKey('PostPage', on_delete=models.CASCADE, related_name='rn_comments')
  
      def save(self, *args, **kwargs):
          if self.user:
              self.user_name = self.user.username
          self.page = PostDetail.objects.get(pk=self.object_pk)
          super(PostComment, self).save(*args, **kwargs)
  </code></pre>
  <p>On Wagtail CMS, if I revise an existing post which has some comments already and publish the revised post again, I thought <strong><code>PostComment.save()</code></strong> should not be triggered any more. However, during my debug, I found it was triggered unexpectedly.</p>
  <p>I guess that I need to fine-tune <strong><code>PostComment.save()</code></strong> to achieve above intention.</p>
  <p>After some researches on StackOverflow,</p>
  <ul>
  <li><strong><a href="https://stackoverflow.com/questions/29127768/identifying-new-model-instance-in-django-save-with-uuid-pk">Identifying new Model Instance in Django Save with UUID pk</a></strong></li>
  <li><strong><a href="https://stackoverflow.com/questions/907695/in-a-django-model-custom-save-method-how-should-you-identify-a-new-object/35647389#35647389">In a django model custom save() method, how should you identify a new object?</a></strong></li>
  </ul>
  <p>I realize that I might need to use <strong><code>PostComment._state.adding</code></strong> and <strong><code>force_insert</code></strong> within the <strong><code>save()</code></strong> to achieve my intention.</p>
  <p>Can anyone show me how should I fine-tune <strong><code>PostComment.save()</code></strong> ?</p>