Display the product of a category only if this one has a unique product in Magento

Observer.php

    /**
     * Display the product of a category only if this one has a unique product
     *
     * Event
     * - catalog_controller_category_init_after
     *
     * @param Varien_Event_Observer $observer
     * @return mixed
     */
    public function initSingleProductRedirect(Varien_Event_Observer $observer)
    {
        /* @var $category Mage_Catalog_Model_Category */
        $category = $observer->getEvent()->getCategory();
        $action = $observer->getEvent()->getControllerAction();

        if ($category->getProductCount() == 1 && $category->getChildrenCount() <= 0) {
            $resource = $category->getResource();
            $sql = $resource->getReadConnection()->select()
                ->from(array('cat' => $resource->getTable('catalog/category_product')), 'product_id')
                ->where('cat.category_id = ?', $category->getId());

            $productId = $resource->getReadConnection()->fetchOne($sql);

            $url = Mage::getModel('catalog/product')->load($productId)->getProductUrl();
            if ($url) {
                return $action->getResponse()->setRedirect($url);
            }
        }
    }