How do you add null constraints to an already existing database backed web application?
Let’s talk about adding integrity to an already existing database backed web application. In this screencast, we’ll talk about null values, which can tend to sneak into any large application that has been around for some time. We’ll go over our philosophy on data integrity and our thoughts on nulls, and what to do about them in an existing app with null constraints.
Active Record Migrations -- Ruby on Rails Guides