CakePHP Questions

My case is: Order HABTM Product. In joining table orders_products I've extra field - quantity.

Now, when editing Order I can add Products and enter quantitiy. Here is what I've done already:

In model Order:

public $hasAndBelongsToMany = array(
	'Product' => array(
		'className' => 'Product',
		'joinTable' => 'orders_products',
		'foreignKey' => 'order_id',
		'associationForeignKey' => 'product_id',
		'unique' => true,
		'with' => 'OrdersProduct'
	)
);

In model Product:

public $hasAndBelongsToMany = array(
	'Order' => array(
		'className' => 'Order',
		'joinTable' => 'orders_products',
		'foreignKey' => 'product_id',
		'associationForeignKey' => 'order_id',
		'unique' => true,
		'with' => 'OrdersProduct'
	)
);

I've created model for joining table OrdersProduct:

class OrdersProduct extends AppModel {
	public $name = 'OrdersProduct';

	public $validate = array(
		'quantity' => array(
			'numeric' => array('rule' => array('notempty'))
		)
	);
}

In orders editing view I've:

<?php echo $this->Form->input("Product.0.OrdersProduct.product_id", array('label' => false, 'options' => $products)); ?>
<?php echo $this->Form->input("Product.0.OrdersProduct.quantity", array('label' => false, 'class' => 'text')); ?>

Now in Orders controller try to save:

	// Form processing
	if (!empty($this->data)) {
		$this->Order->OrdersProduct->set($this->data);
		debug($this->Order->OrdersProduct->validates());

		if ($this->Order->saveAll($this->data)) {
			// Do some stuff
		}

And $this->Order->OrdersProduct->validates() always returns TRUE, no matter if I enter quantity or not. All the data saves perfect, only validation is problem. What I'm missing?