How magento redirect the checkout page to "edit profile" page

Question

I'm working on a onepage checkout,

In the original version of magento, when I erase the firstname of a user in database, and load checkout page, it will automatically redirect to edit profile page to fill the firstname field.

I want to use this feature, Can you suggest some approches?

Ofcource , if anyone can explain the magento way, I would appreciate that more.

UPDATE :

I want an approach to add another field for user, just like the firstname field. So that whenever that field is empty, the checkout page should redirect to "edit profile" page.

Answer 1

You can add custom field for Customer by following this link https://magento.stackexchange.com/questions/5905/adding-custom-attribute-to-customer-registration-form or the tutorial http://www.magentocommerce.com/wiki/5_-_modules_and_development/customers_and_accounts/registration_fields

Then add validation condition for your attribute just like other validations in Customer Model file app/code/core/Mage/Customer/Mode/Customer.php. Arround line number 792 look for method public function validate()