One time script to set custom attribute for ~1m customers - memory leak

Question

I'm trying to run a one-time script (in a custom module upgrade) to set a new attribute last_order_date based on the date of that customer's last order.

I have this script:

$users = mage::getModel('customer/customer')->getCollection()
           ->addAttributeToSelect('last_order_date');

foreach ($users as $user){
   $lastDate = $user->getData('last_order_date');
   if(empty($lastDate)){
       $orderCollection = Mage::getModel('sales/order')->getCollection()
            ->addFilter('customer_id', $user->getId())
            ->setOrder('created_at', Varien_Data_Collection_Db::SORT_ORDER_DESC)
        ;
        $numberOfOrders = $orderCollection->count();
        if($numberOfOrders!=0){
            $newestOrder = $orderCollection->getFirstItem();
            $lastOrderDate = $newestOrder->getCreatedAt();
            $user->setData('last_order_date', $lastOrderDate);
            $user->save();
        }
   }
}

But it's causing a memory leak:

Fatal error: Allowed memory size of 1073741824 bytes exhausted (tried to allocate 32 bytes) in [..]/web-root/lib/Zend/Db/Adapter/Pdo/Abstract.php on line 292

Which I understand is due to the looped calls to Mage::getModel('sales/order').

How do I achieve this without the repeated calls?

Answer 1

require_once 'app/Mage.php';
Mage::app();

$current_page = isset($_GET['current_page']) ? 1 : $_GET['current_page'];

$users = mage::getModel('customer/customer')->getCollection()
           ->addAttributeToSelect('last_order_date')
           ->setPageSize(100)
           ->setCurPage($current_page);

if($users->count() == 0){
    die('done');
}

foreach ($users as $user){
   $lastDate = $user->getData('last_order_date');
   if(empty($lastDate)){
       $orderCollection = Mage::getModel('sales/order')->getCollection()
            ->addFilter('customer_id', $user->getId())
            ->setOrder('created_at', Varien_Data_Collection_Db::SORT_ORDER_DESC)
        ;
        $numberOfOrders = $orderCollection->count();
        if($numberOfOrders!=0){
            $newestOrder = $orderCollection->getFirstItem();
            $lastOrderDate = $newestOrder->getCreatedAt();
            $user->setData('last_order_date', $lastOrderDate);
            $user->save();
        }
   }
}

$current_page++;

//go to next page
$redirect_url = "http://yoururlhere/set_last_order_date.php?current_page=" . $current_page;
?>
<script type="text/javascript">
    setTimeout("location.href = '<?php echo $redirect_url;?>';", 50);
</script>

Answer 2

Added to gurutheme answer a huge performance server is to use saveAttribute instead of save

So replace the following

$user->setData('last_order_date', $lastOrderDate);
        $user->save();

With:

$user->setData('last_order_date', $lastOrderDate);
        $user->getResource()->saveAttribute($user,'last_order_date');

This way you will only save one attribute instead of saving every attribute of the customer model.