model
class Comment(models.Model):

    user = models.ForeignKey(User, on_delete=models.CASCADE)
    post = models.ForeignKey(Post, on_delete=models.CASCADE)
    content = models.TextField(max_length=100000, verbose_name='')
    timestamp = models.DateTimeField(auto_now_add=True,verbose_name='Время', null=True, blank=True)
    likes = models.ManyToManyField(User, related_name='likes', blank=True)

    reply = models.ForeignKey('Comment', on_delete=models.CASCADE, null=True, related_name='replies')  <---------
view
class PostDetailView(FormMixin, DetailView):
    model = Post
    template_name = 'post/post_detail.html'
    form_class = CommentForm
    paginate_by = 2


    def get_success_url(self):
        return reverse('post-detail', kwargs={'pk': self.object.id})

    def get_context_data(self, **kwargs):

        context = super(PostDetailView, self).get_context_data(**kwargs)
        context['comments'] = Comment.objects.filter(post=self.object)
        context['form_comment'] = CommentForm(initial={'post': self.object})
        return context

    def post(self, request, *args, **kwargs):
        self.object = self.get_object()
        form = self.get_form()
        if form.is_valid():
            return self.form_valid(form)
        else:
            return self.form_invalid(form)

    def form_valid(self, form):
        form.instance.user = self.request.user
        form.instance.post = self.object
        form.save()
        return super(PostDetailView, self).form_valid(form)