title: save() missing 1 required positional argument: 'self' Error while using Django Signal Dispatcher
body: <p><strong>models.py</strong></p>
  
  <pre><code>class es_event(models.Model):
      #some attributes
      reg_applicants = models.FilePathField(path=None)
      ev_admin =  models.ForeignKey('es_user',related_name='events',on_delete=None)
      slug = models.SlugField(max_length=250)
  
      def save(self, *args, **kwargs):
          self.slug = slugify(self.ev_name)
          return super(es_event, self).save(*args, **kwargs)
  
      def get_absolute_url(self):
              return reverse('event_detail', kwargs={'id': self.id, 'slug': self.slug })
  </code></pre>
  
  <p><strong>views.py</strong></p>
  
  <pre><code>class CreateEventView(LoginRequiredMixin,CreateView):
      login_url = "/login/"
      model = es_event
      fields = ['ev_name','ev_venue','ev_description','ev_date','registrant_name','registrant_age','registrant_phone','registrant_email','registrant_institution']
      def form_valid(self, form):
          form.instance.ev_admin = self.request.user.es_user
          return super(CreateEventView, self).form_valid(form)
  </code></pre>
  
  <p>Now when a new row is added to es_event. I want to set the <strong>reg_applicants</strong> field with a path(including filename) </p>
  
  <p>eg: if <strong>es_event.id</strong> is 5 then filename should be <strong>Registered_5.csv</strong></p>
  
  <p>To do that I created this Signal in <strong>models.py</strong></p>
  
  <pre><code>@receiver(post_save, sender=es_event)
  def set_csv_path(sender, **kwargs):
      rel_path = "reg_csv/Registered_{}.csv".format(sender.id)
      path = os.path.join(settings.MEDIA_ROOT,rel_path)
      sender.reg_applicants = path
      sender.save()# error statement
  </code></pre>
  
  <p>which gives me this error</p>
  
  <pre><code>save() missing 1 required positional argument: 'self'
  </code></pre>
  
  <p>I think there is something wrong with the signal dispatcher function <strong>set_csv_path()</strong>. I don't know what it is</p>