Table Structure

bootstrap.php

use lithium\data\Connections;

Connections::get("default")->applyFilter("_execute", function($self, $params, $chain) { 
	var_dump($params['sql']); 
	return $chain->next($self, $params, $chain); }
)

Comments.php

<?php

namespace app\models;

class Comments extends \lithium\data\Model {

        public $validates = array();

        public $belongsTo = array('Posts','Users');

        protected $_schema = array(
            'id'        => array('type' => 'integer', 'key' => 'primary'),
            'user_id' => array('type'=>'integer'),
            'post_id' => array('type'=>'integer'),
            'comment'   => array('type' => 'text'),
            'updated'   => array('type' => 'datetime'),
            'status' => array('type' => 'integer')
        );
}

?>

lithium_blog.sql

--
-- Table structure for table `comments`
--

DROP TABLE IF EXISTS `comments`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `comments` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `user_id` int(11) NOT NULL,
  `post_id` int(11) NOT NULL,
  `comment` text NOT NULL,
  `updated` datetime NOT NULL,
  `status` int(11) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=9 DEFAULT CHARSET=latin1;
/*!40101 SET character_set_client = @saved_cs_client */;

--
-- Table structure for table `users`
--

DROP TABLE IF EXISTS `posts`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `posts` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `title` varchar(100) NOT NULL,
  `body` text NOT NULL,
  `url` varchar(100) NOT NULL,
  `user_id` int(11) NOT NULL DEFAULT '0',
  `created` datetime NOT NULL,
  `updated` datetime NOT NULL,
  `status` int(11) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=latin1 COMMENT='Posts table';
/*!40101 SET character_set_client = @saved_cs_client */;

--
-- Table structure for table `users`
--

DROP TABLE IF EXISTS `users`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `users` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `username` varchar(50) NOT NULL,
  `password` varchar(50) NOT NULL,
  `fullname` varchar(100) NOT NULL,
  `url` varchar(50) DEFAULT NULL,
  `address_id` int(11) DEFAULT NULL,
  `email` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=latin1 COMMENT='List of users of website';
/*!40101 SET character_set_client = @saved_cs_client */;

Posts.php

<?php

namespace app\models;

class Posts extends \lithium\data\Model {

	public $validates = array(
		'title' => array('notEmpty', 'message' => 'You need a title'),
		'body' => array('notEmpty', 'message' => 'You need a body')
        );

        public $hasMany = array(
		'Comments'=>array(
			'key' => array('id'=>'post_id'),
			'conditions' => array('Comments.status' => '1'),
			'order' => 'Comments.created DESC',
		)
		//'Tagging'=>array('keys' => array('id'=>'post_id'))
	);

	public $belongsTo = array('Users');

/*
	public $belongsTo = array(
		'Users' => array(
			'key' => array('user_id'=>'id')
		)
	);
*/

        protected $_schema = array(
            'id'        => array( 'type'=>'integer', 'key'=> 'primary'),
            'title'     => array('type'=>'string'),
            'body'      => array('type'=>'text'),
	    'url'     => array('type'=>'string'),
	    'user_id' => array('type'=>'integer'),
            'created'   => array('type'=>'datetime'),
            'updated'   => array('type'=>'datetime'),
	    'status' => array('type'=>'integer')
        );

}

?>

Users.php

<?php

namespace app\models;


class Users extends \lithium\data\Model {

	public $validates = array(
    		'fullname' => 'Please enter a full name',
    		'username' => 'Please enter a user name',
    		'email' => array(
        		array('notEmpty', 'message' => 'email is empty'),
        		array('email', 'message' => 'email is not valid'),
    		)
	);

	public $hasMany = array(
		'Posts' => array('key'=>array('id'=>'post_id')),
		'Comments' => array('key'=>array('id'=>'comment_id'))
	);

	protected $_schema = array(
		'id' => array('type'=>'integer', 'key'=>'primary'),
		'username' => array('type'=>'string'),
		'password' => array('type'=>'string'),
		'fullname' => array('type'=>'string'),
		'url' => array('type'=>'string'),
		'address_id' => array('type'=>'integer'),
		'email'=>array('type'=>'integer')
	);

}

Users::applyFilter('save', function($self, $params, $chain){
    $record = $params['entity'];
    if (!$record->id) {
        $record->password = \lithium\util\String::hash($record->password);
    }
    if (!empty($params['data'])) {
	$params['data']['password'] = \lithium\util\String::hash($params['data']['password']);
        $record->set($params['data']);
    }
    $params['entity'] = $record;
    return $chain->next($self, $params, $chain);
});

?>

view.html.php

        public function view() {
		

//	Select all fields from the both table

		$post = Posts::find('first', array(
				'conditions'=> array('Posts.id'=>$this->request->id),
				'with'=>array('Users')));

//Don't want to fetch certain datas like password, credit card like stuffs ?, use fields :)		
/*
		$post = Posts::find('first', array(
				'conditions'=> array('Posts.id'=>$this->request->id),
				'fields'=>array(
					'Posts'=>array('id','title','body','created'),
					'Users'=>array('id','fullname'),),
				'with'=>array('Users')));
*/

		if (!$post) {
			return $this->redirect('Posts::index');
		}
		$comments = Comments::find('all',array(
					'conditions'=>array('post_id'=>$this->request->id),
					'with'=>array('Users')
				));

/*
//Helpful when you want to dump the data ->data();
		echo '<pre>';
		print_r( $post->data() );
		print_r( $comments->data() );
		echo '</pre>';
*/
		return compact('post', 'comments');
	}

Little proposal:
For portability iwould'nt it be nicer if we use $model->exists() instead of checking against the id? (mongo: _id ...)

Yes @dgAlien good one :) .