Array of product ids for a given category (ignoring is_anchor)

Question

When grabbing the product id's from a category where the is_anchor attribute is set we get the complete collection. Including products from underlying categories.

Question: How can we get a list/array of product id's for any given category id that has the is_anchor attribute set - where the collection contains only the real products from that category only (and not also products from underlying categories)

This is what we have to date. But I am sure this is not the fastest method.

$category = new Varien_Object(array('id'=>$_cat));
$positions = Mage::getResourceModel('catalog/category')->getProductsPosition($category);
$includeIDs = array_keys($positions);

Reference: Array of product ids for a given category (respecting is_anchor)

Answer 1

I think the fastest way (performance) to get only an array containing the product ids for a category is to build your own query such as :

$resource = Mage::getResourceModel('core/resource');
$adapter = $resource->getReadConnection();
$product_ids = $adapter->fetchCol(
    $adapter->select()
        ->from(
            $resource->getTable('catalog/category_product'),
            array('product_id')
        )
        ->where('category_id = ?', $cat)
);

Answer 2

I think the only alternative is to join catalog_category_product into the product collection