passing ID from one controller to another

hi guys,... 

what I'm doing is creating comments for every blog post,...and I need to pass blog post ID to a comments table and having problem: Undefined variable id in CommentsComtroller.php 

this is my Model: 

<?php

class Comment extends AppModel {

   public $name = 'Comment';

   public $belongsTo = array('User', 'Post');

}

?>

and this is my CommentsController.php action 

<?php

class CommentsController extends AppController {

  var $name = 'Comments';

    public $uses = array('Post', 'Comment', 'User');

    public $helpers = array('Html', 'Form');

    public $components = array('Session');

public function isAuthorized($user) {

    if ($this->action === 'add') {

        return true;

    }

    return parent::isAuthorized($user);

}

    public function add() {

    if(!($post = $this->Post->findById($id)))

        if ($this->request->is('post')) {

        $this->request->data['Comment']['user_id'] = $this->Auth->user('id');

        $this->request->data['Comment']['post_id'] = $post['Post']['id'];  // HOW CAN I IDENTIFY POST ID FROM PostsController.php    ????? //

            $this->Comment->create();

            if ($this->Comment->save($this->request->data)) {

                $this->Session->setFlash('Your comment has been saved.');

                $this->redirect(array('controller' => 'posts', 'action' => 'index'));

            } else {

                $this->Session->setFlash('Unable to add your post.');

            }

        }

    }

can anyone help...? 

thanks in advance 

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