问magento:致命错误:调用未定义的方法Varien_Data_Collection::addFieldToFilter()

EN

class Nextshopping_Shipment_Model_Mysql4_Shipment_Collection extends     Mage_Core_Model_Mysql4_Collection_Abstract
{
    public function loadProductForShipment(){

    $coll = new  Varien_Data_Collection();

    $shipment = Mage::getModel('shipment/shipment')->getResourceCollection();
    $order_item_id = array();
    if($shipment){
        foreach ($shipment as $item){
            $order_item_id[] = $item->getOrderItemId();
        }
    }
    $collection = Mage::getModel('sales/order_item')->getCollection();

    if($order_item_id){
        $collection = $collection ->addFieldToFilter('item_id',array('nin'=>$order_item_id));
    }
    $collection->addAttributeToSelect('item_id');
    $collection->addAttributeToSelect('product_id');
    $collection->addAttributeToSelect('name');
    $collection->addAttributeToSelect('sku');
    $collection->addAttributeToSelect('qty_ordered');


    $collection->getSelect()
    ->join('sales_flat_order','main_table.order_id = sales_flat_order.entity_id AND sales_flat_order.state = "complete"',
            array('state','status as order_status','increment_id as order_increment_id'))
            ->join('sales_flat_invoice','sales_flat_invoice.order_id = sales_flat_order.entity_id',
                    array('entity_id as invoice_id'))
                    ->join('sales_flat_order_payment','sales_flat_order_payment.entity_id = sales_flat_order.entity_id AND method != "cashondelivery"',
                            array('method'))
                            ->join('sales_flat_shipment','sales_flat_shipment.order_id = sales_flat_order.entity_id',
                                    array('created_at','updated_at'));
    if($collection){
        foreach ($collection as $item){
            $object = new Varien_Object();

            $object->setData($item->getData());

            $object->setData('qty_shipmented',0);

            if(strtotime($item['created_at']) + 3600*24*3 < time()){
                $object->setData('more_than_three',$item['qty_ordered']);
            }else{
                $object->setData('more_than_three',0);
            }

            if(strtotime($item['created_at']) +3600*24*5 < time()){
                $object->setData('more_than_five',$item['qty_ordered']);
            }else{
                $object->setData('more_than_five',0);
            }

            if(strtotime($item['created_at']) +3600*24*7 < time()){
                $object->setData('more_than_seven',$item['qty_ordered']);
            }else{
                $object->setData('more_than_seven',0);
            }
            $coll->addItem($object);
        }
    }
    //$coll = Mage::getModel('sales/order_item')->getResourceCollection();
    return $coll;
}


 protected function _prepareCollection()
{
  $collection = Mage::getModel('shipment/shipment')->getCollection()->loadProductForShipment();
   $this->setCollection($collection);
   return parent::_prepareCollection();
}