Hi,
No. The product_category_id column is in the hikashop_product_category table.
But the get function of class.product only loads the data in the hikashop_product table which doesn't contain that column.
So it's normal this doesn't work.
You can do it like this:
$productClass = hikashop_get('class.product');
foreach ($order->cart->full_products as $product) {
$categories = $productClass->getCategories($product->product_id);
foreach($categories as $category_id) {
echo $category_id;
}
}
Note that one product can be linked to several categories.