What is the correct way to update a value to be NULL on a Magento model?

Question

I am having an issue getting a custom model's value to update to NULL.

$model = Mage::getModel('custom/model')->load($id);
$model->setCustomValue(NULL);
$model->save();

This code does not update the field custom_value in the database, and the database value accepts NULL values (Manually setting NULL in the database works fine).

EDIT: As Tim mentions below using Zend_Db_Expr("NULL") does set the value to null, could anyone explain why this is required in Magento and not just simply a php NULL ?

Answer 1

This is, as so often a isset() vs array_key_exists() problem, when I see this correctly.

I didn't try it, but from code reading I would say, the value is checked here:

\Mage_Eav_Model_Entity_Abstract::_saveAttribute
if (!isset($this->_attributeValuesToSave[$table])) {
    $this->_attributeValuesToSave[$table] = array();
}

and because isset return false on a set array key which is NULL, your value is overwritten.