Get product attribute in cart with flat products enabled

Question

I am using a forced_shipping attribute and the below code in Mage\Shipping\Model\Carrier\Flatrate.php to restrict shipping method for items with the forced_shipping attribute set to Yes.

$items = Mage::getModel('checkout/session')->getQuote()->getAllItems();
$forcedShipping = false;
foreach ( $items as $item ) {
    $_products = Mage::getModel('catalog/product')->getCollection()->addAttributeToFilter('sku', $item->getSku())->addAttributeToSelect('forced_shipping', 'inner');
    foreach ( $_products as $_product) {
        if ( $_product->getForced_shipping() == 1 ) {
            $forcedShipping = true;
        }
    }
}
if ( !$forcedShipping ) {
    return false;
}

While this is working great with flat products disabled, it will fail once I enable them. How do I need to modify my code to make it work with flat products, is it possible at all?

Answer 1

Turns out the setting Used on Product Page does not include an attribute in the flat table. I had to set Used in Product Listing to Yes to make it work.

Apparently any of these settings will populate the flat table with an attribute:

Use in Layered Navigation, Used in Product Listing, Used for Sorting in Product Listing