Модель
class Comments(models.Model):
    body = models.TextField(max_length=200)
    comment_author = models.ForeignKey(CustomUser,related_name='author_comments',on_delete=models.PROTECT)
    post = models.ForeignKey(Post,related_name='comments',on_delete=models.PROTECT)

Вьюха создания комментария
class CommentCreate(CreateView):
    model = Comments
    fields = ['body']
    template_name = 'blog/comment_create.html'
    success_url = '/blog/'

    def form_valid(self, form):
        form.instance.comment_author = self.request.user
        form.instance.post = Post.objects.get(pk=self.pk)  # TODO Узнай , как сделать поиск текужего поста
        return super().form_valid(form)