/**
* set collection object
*
* @param Varien_Data_Collection $collection
*/
public function setCollection($collection)
{
if (Mage::helper('catalog')->isModuleEnabled('Mage_CatalogInventory')) {
$select = $collection->getSelect();
$fromParts = $select->getPart(Zend_Db_Select::FROM);
if (isset($fromParts['at_qty'])) {
unset($fromParts['at_qty']);
$select->reset(Zend_Db_Select::FROM);
$select->setPart(Zend_Db_Select::FROM, $fromParts);
$subquery = $collection->getConnection()->select()->from(array('at_sub_qty' => $collection->getResource()->getTable('cataloginventory/stock_item')), array('product_id'))->columns(new Zend_Db_Expr("SUM(at_sub_qty.qty) AS qty"))->group('at_sub_qty.product_id');
$select->joinLeft(array('at_qty' => $subquery), 'at_qty.product_id=e.entity_id', array());
}
}
parent::setCollection($collection);
}