Can't save empty string into field value - magento

Question

I need to change value of NULL-field to empty string (in sales_flat_order table).
I tried this code:
$orders = Mage::getModel('sales/order')->getCollection()
->addAttributeToFilter('created_at', array('to' => $endDate))
->addAttributeToFilter('status', array('eq' => Mage_Sales_Model_Order::STATE_COMPLETE));
foreach ($orders as $order) {
$comment = $order->getCustomerNote();
if (is_null($comment)) {
$order->setData('customer_note', '');
try {
$order->save();
} catch (Exception $e){
echo $e->getMessage(); exit;
}
}
}
But in base I see still NULL value in this filed.
If I change empty string to any non-empty it works fine.
How can I update NULL value field to empty string?
Thanks.

Answer 1

The empty string is set to null in Varien_Db_Adapter_Pdo_Mysql::prepareColumnValue():
case 'varchar':
case 'mediumtext':
case 'text':
case 'longtext':
$value = (string)$value;
if ($column['NULLABLE'] && $value == '') {
$value = null;
}
break;
To reset the column value to empty string, add this in your resource model:
protected function _prepareDataForSave(Mage_Core_Model_Abstract $object)
{
$data = parent::_prepareDataForSave($object);
if ($object->getData('your_column_name') === '') {
$data['your_column_name'] = '';
}
return $data;
}

Answer 2

I tried and it's normal.
$order->setData('customer_note', new Zend_Db_Expr(''));
try {
$order->save();
} catch (Exception $e){
echo $e->getMessage(); exit;
}