Mage::getResourceMode

$collection = Mage::getResourceModel( $this ->_getCollectionClass());           //we changed mysql query, we added inner join to order item table          $collection ->join( 'sales/order_item' , 'order_id=entity_id' , array ( 'name' => 'name' , 'sku' => 'sku' , 'qty_ordered' => 'qty_ordered' ), null, 'left' );          $this ->setCollection( $collection );