[SOLVED]How to save or update master-detail?

Dear experts,

How to save or update master-detail in correct ways?

I've added Post and Comment model. But confuse to save or update comments together with a post.

class Posts extends Model
{
    /**
     * @var integer
     */
    public $id;

    /**
     * @var string
     */
    public $date;

    /**
     * @var string
     */
    public $content;

    public function initialize()
    {   
        $this->hasMany('id', 'MM\Models\Comments', 'postsId', array(
            'alias' => 'comments',
            'foreignKey' => array(
                'message' => 'Comments cannot be deleted because it has link'
            )
        ));
    }
}

class Comments extends Model
{
    /**
     * @var integer
     */
    public $id;

    /**
     * @var integer
     */
    public $postsId;

    /**
     * @var string
     */
    public $body;

    public function initialize()
    {   
        $this->belongsTo('postsId', 'MM\Models\Posts', 'id', array(
            'alias' => 'post',
            'reusable' => true
        )); 

    }
}

Here's inside the controller's save function:

$post = new Posts();
$post->date = '2013-09-20';
$post->content = 'Some post here';

$comment1 = new Comments();
$comment1->body = "Some comment 1";

$comment2 = new Comments();
$comment2->body = "Some comment 2";

//
// Now how to append all comments to the post and saving it all together?
// 
// $post->comment->add($comment1) ? maybe?
//

$post->save();

I know this is very basic question and this answer is might help another new beginners like me :)

Thank you so much.

Jim