Mine

    function add($parent_id = null)
    {
        $this->set('parent_id', $parent_id);
        # Is a post being sent?
        if (!empty($this->data))
        {
            # if there is a parent, find ancestor, too
            if (isset($this->data['Post']['parent_id']) &&
                $parent_id = (int)$this->data['Post']['parent_id'])
            {
                $parent_post = $this->Post->find('id = '.$parent_id);
                # if parent has no ancestor, parent *is* the ancestor, too
                if ($parent_post['Post']['ancestor_id'] === null) {
                    $this->data['Post']['ancestor_id'] = $parent_id;
                } else {
                    # otherwise, inherit parent's ancestor.
                    $this->data['Post']['ancestor_id'] =
                        $parent_post['Post']['ancestor_id'];
                }
            }
            if ($this->Post->save($this->data))
            {
                $this->redirect('/');
                exit;
            }
        }
        # Otherwise, read in the parent post's data if necessary.
        # This will be displayed on the "You're replying to..." page.
        else
        {
            if ($parent_id !== null)
            {
                $this->Post->id = $parent_id;
                $parent_post = $this->Post->read();
                $this->set('parent_post', $parent_post);
            }
            else
            {
                $this->set('parent_post', null);
            }
        }
    }