Get Number of Orders per Product - magento

Question

I'm trying to build an array of orders => products that I can use in reporting/updating attributes. The format I'm going for is:
//$orders[<number of orders>] = <array of product ids with this many orders>
$orders = array(
1 => array(1, 2, 3),
2 => array(4, 5)
//etc
);
So far the best I can do is
$productCollection = Mage::getModel('catalog/product')
->addAttributeToSelect("sku")
->getCollection();
$orders = array();
foreach ($productCollection as $product) {
$ordered = Mage::getResourceModel('reports/product_collection')
->addOrderedQty()
->addAttributeToFilter('sku', $product->getSku())
->setOrder('ordered_qty', 'desc')
->getFirstItem();
$qtyOrdered = $ordered->getOrderedQty();
$total = $this->_counter - (int)(!$ordered ? 0 : $qtyOrdered);
if (!is_array($orders[$total])) {
$orders[$total] = array();
}
$orders[$total][] = $product->getId();
}
But this is obviously using a lot of resources, loading the entire product collection.
I think I can get by just using
$ordered = Mage::getResourceModel('reports/product_collection')
->addOrderedQty();
But I'm having trouble returning/iterating through the results. How do I extract the information I'm looking for?

Answer 1

This should be better.
$orderItems = Mage::getResourceModel('sales/order_item_collection');
$result = array();
foreach ($orderItems as $item) {
$result[$item->getOrderId()][] = $item->getProductId();
}
print_r($result);
NOTE: take care about hidden order item (Ex: simple, configurable product)

Answer 2

$ordered = Mage::getResourceModel('reports/product_collection')
->addOrderedQty();
foreach($ordered as $order) {
$qtyOrdered = (int)$order->getOrderedQty();
if (!is_array($totals[$qtyOrdered])) {
$totals[$qtyOrdered] = array();
}
$totals[$qtyOrdered][] = $order->getEntityId();
}
Works, but it seems to be returning product Ids that don't exists anymore.