Storing related records: Object of class Multiple\Frontend\Models\Attributes could not be converted to string

Hello,

I'm trying to run a example in the Storing related records for 3 table but all I get is an error message reading Object of class Attributes could not be converted to string.

--
-- Table structure for table `entity_main`
--

CREATE TABLE `entity_main` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `title` varchar(255) COLLATE utf8_persian_ci NOT NULL,
  `origin` int(11) NOT NULL,
  `destination` int(11) NOT NULL,
  `date_start` int(11) NOT NULL,
  `date_end` int(11) NOT NULL,
  `description` text COLLATE utf8_persian_ci NOT NULL,
  `agency_id` int(11) NOT NULL,
  `los` varchar(255) COLLATE utf8_persian_ci NOT NULL,
  `min_price` int(11) NOT NULL,
  `discount_percent` int(11) NOT NULL,
  `discount_price` int(11) NOT NULL,
  `date` int(11) NOT NULL,
  `registrar` int(11) NOT NULL,
  `city_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `fk_entity_tours_city1_idx` (`city_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_persian_ci AUTO_INCREMENT=41 ;

--
-- Table structure for table `attributes`
--

CREATE TABLE `attributes` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `title` varchar(255) COLLATE utf8_persian_ci NOT NULL,
  `entity_kind` varchar(255) COLLATE utf8_persian_ci NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_persian_ci AUTO_INCREMENT=2 ;

--
-- Table structure for table `attribute_values`
--

CREATE TABLE `attribute_values` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `value` varchar(255) COLLATE utf8_persian_ci NOT NULL,
  `attribute_id` int(11) NOT NULL,
  `entity_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `fk_attribute_values_attributes_idx` (`attribute_id`),
  KEY `fk_attribute_values_entity_tours1_idx` (`entity_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_persian_ci AUTO_INCREMENT=11 ;

// model 1
namespace Multiple\Frontend\Models;

use Phalcon\Mvc\Model;

class Entity extends Model
{

    public $id;

    public $title;

    public $origin;

    public $destination;

    public $date_start;

    public $date_end;

    public $description;

    public $agency_id;

    public $los;

    public $min_price;

    public $date;

    public $registrar;

    public $city_id;

    public $discount_price;

    public function initialize()
    {
        $this->setSource("entity_tour");
        $this->hasMany("id", "Attribute_values", "entity_id");
    }

    public function getSource()
    {
        return "entity_main";
    }

}

// model 2
namespace Multiple\Frontend\Models;

use Phalcon\Mvc\Model;

class Attributes extends Model
{

    public $id;

    public $title;

    public $entity_kind;

    public function initialize()
    {
        $this->hasMany("id", "Attribute_values", "attribute_id");
    }

}

//model 3
namespace Multiple\Frontend\Models;

use Phalcon\Mvc\Model;

class Attribute_values extends Model
{

    public $id;

    public $value;

    public $attribute_id;

    public $entity_id;

    public function initialize()
    {
        $this->belongsTo("entity_id", "Entity", "id");
        $this->belongsTo("attribute_id", "Attributes", "id");
    }

}

// action and run this
public function createAction()
    {

        // Create an Entity 
        $entity = new models\Entity();
        $entity->title = 'test title';
        $entity->city_id = '2';
        $entity->origin = '2';
        $entity->destination = '2';
        $entity->date_start = '2';
        $entity->date_end = '2';
        $entity->description = '2';
        $entity->destination = '2';
        $entity->agency_id = '2';
        $entity->los = '2';
        $entity->min_price = '2';
        $entity->date = '2';
        $entity->discount_price = '2';
        $entity->discount_percent = '2';
        $entity->registrar = '2';

        // Create an attribute
        $attributes = new models\Attributes();
        $attributes->title = 'test';
        $attributes->entity_kind = 'entity'; 

        // Create an attribute_value
        $attribute_values = new models\Attribute_values();
        $attribute_values->value = 'test';
        $attribute_values->attribute_id = $attributes; 
        $attribute_values->entity_id = $entity; 

        //Save all records
        $attribute_values->save();

    }

after run it I have this error

Object of class Multiple\Frontend\Models\Attributes could not be converted to string Thanks