Saving Order Collection

Question

I'm trying to run an update script to a module as i've changed the way our warehouse module works. I need to update all orders in a collection of orders if it meets a certain requirement. I know that i can save each order as it goes through, but i'd prefer to save the entire collection once it has updated all values. $collection->save(); causes the page to go white with no errors thrown. Can anyone see where i'm going wrong. Here is my script.

    <?php

$installer = $this;

$_last_export_date = Mage::getStoreConfig('warehouse/runtime/sales_last_export');

$collection = Mage::getModel('sales/order')->getCollection()->addAttributeToSelect('*');

foreach($collection as $order) {
    if($order->getCreatedAt() < $_last_export_date) {
       $order->setPointbidExported(1);
    }
}

//$collection->walk('save');

$collection->save();

EDIT ==========================

Thank you Marius for your response, I don't have the authority to answer my own question at this time so a comment will have to do.

I found this error in my apache logs "Found an error : PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 235929601 bytes)" So i don't think there was anything wrong with my method. For those interested my solution was to just directly query the database using sql. See below :

<?php

$installer = $this;

$_last_export_date = Mage::getStoreConfig('warehouse/runtime/sales_last_export');

$write = Mage::getSingleton('core/resource')->getConnection('core_write');
$write->query('UPDATE sales_flat_order SET pointbid_exported = 1 WHERE created_at < '.'"'.$_last_export_date.'"'); 

This then wont exceed the allowed memory size as it doesn't use PHP to process.

Answer 1

Try this.

$_last_export_date = Mage::getStoreConfig('warehouse/runtime/sales_last_export');
$collection = Mage::getModel('sales/order')->getCollection()->addAttributeToSelect('*');
$resourceModel = Mage::getResourceModel('sales/order');
foreach($collection as $order) {
    if($order->getCreatedAt() < $_last_export_date) {
       $order->setPointbidExported(1);
       $resourceModel->saveAttribute($order, 'pointbid_exported');
    }
}