Cactiverecord Rules To Check If Data Already Exists

Dear all,

I am creating new datasets but currently I don´t know how to check it data already exists in the database. I have read the the save() should automatically check the rules function. Is that true?
I think it is not working because the line

array('id_os', 'exists', 'on'=>'insert_post'),

is not good.

I have

	public function actionCreate()
	{
		$model=new Operatingsystem;

		// Uncomment the following line if AJAX validation is needed
		// $this->performAjaxValidation($model);

		if(isset($_POST['Operatingsystem']))
		{
			$model->attributes=$_POST['Operatingsystem'];
                        if($model->save())
				$this->redirect(array('view','id'=>$model->id_os));
		}

		$this->render('create',array(
			'model'=>$model,
		));
	}

and this rules function.

	public function rules()
	{
		// NOTE: you should only define rules for those attributes that
		// will receive user inputs.
		return array(
			array('id_os', 'required'),
			array('id_os', 'length', 'max'=>20),
			array('os_name', 'length', 'max'=>45),
			// The following rule is used by search().
			// Please remove those attributes that should not be searched.
			array('id_os, os_name', 'safe', 'on'=>'search'),
                        array('id_os', 'exists', 'on'=>'insert_post'),
		);
	}