In reply to https://www.facebook.com/groups/cakebrasil/permalink/955837084447545/

index.php

<?php
// Controller
public function add_cart($id = null)
{
    $product = $this->Product->find(
        'first',
        array(
            'recursive' => -1,
            'fields' => array(
                'Product.id',
                'Product.description',
                'Product.price',
                'Product.quantity',
                'Product.image'
            ),
            'conditions' => array(
                'Product.id' => $id
            )
        )
    );

    if (!$product) {
        throw new NotFoundException('Compra Inválida');
    }

    $this->saveProduct($product);

    $this->Session->setFlash('Produto adicionado ao carrinho.', 'flash_success');
    return $this->redirect(
        array(
            'action' => 'cart',
            $this->makeCart()
        )
    );
}

private function saveProduct(array $product)
{
    $cart = CakeSession::read('cart');

    if (!$cart) {
        $cart = array();
    }

    $cart[] = $product;

    return CakeSession::write('cart', $cart);
}