Magento how to add new attribute to sales/order model?

Question

I want to add one attribute to sales/order, that is the Mage_Sales_Model_Order, to store some extra data.

I found the resource of sales/order uses normal table. And when I save an order, it only saves the fields that match the columns in the table.

What is the right way to add this attribute?

Answer 1

I would recommend a hybrid of a few of the answers below. You definitly want to put this code in a install script via config xml but your going to want to use the Mage_Eav_Model_Entity_Setup as your setup class if you want to leverage the addAttribute function. So your config xml will look something like this...

<config>
...
    <resources>
        <modulename_setup>
            <setup>
                <module>Your_Module</module>
                <class>Mage_Eav_Model_Entity_Setup</class>
            </setup>               
        </modulename_setup>           
    </resources>
...
</config>

and your install script should look something like this

$installer = $this;
/* @var $installer Mage_Catalog_Model_Resource_Eav_Mysql4_Setup */

$installer->startSetup();

$installer->addAttribute('sales_order', 'attributename', array(
            'group'             => 'General',
            'label'             => 'Label frontend',
            'note'              => '',
            'type'              => 'string',    //backend_type
            'input'             => 'text', //frontend_input
            'frontend_class'    => '',
            'source'            => '',
            'backend'           => '',
            'frontend'          => '',
            'global'            => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_WEBSITE,
            'required'          => false,
            'visible_on_front'  => false,
            'apply_to'          => 'simple',
            'is_configurable'   => false,
            'used_in_product_listing'   => false,
            'sort_order'        => 5,
        ));

$installer->endSetup();

Keep in mind this is untested code, and you might need to tweak some of the attribute options to get it to function the way you need it to.