views.py
class CommentCreateView(CreateView):
    model = Comment
    template_name = 'post/create_comment.html'
    form_class = CommentForm
    success_url = reverse_lazy('post-detail')

    def form_valid(self, form):
        form.instance.user = self.request.user
        return super(CommentCreateView, self).form_valid(form)

model.py

class Comment(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE, related_name='comments')
    post = models.ForeignKey(Post, on_delete=models.CASCADE)
    content = models.TextField()
    timestamp = models.DateTimeField(auto_now_add=True)
    reply = models.ForeignKey('self', on_delete=models.CASCADE, null=True, blank=True, related_name='replies')

    def __str__(self):
        return 'Comment by {}'.format(self.user)
urls.py

path('post/add_comment/', CommentCreateView.as_view(), name='create_comment'),

error:
IntegrityError at /post/add_comment/
NOT NULL constraint failed: post_comment.post_id