Say for example we have products and a business rule that every product must have a category. Before ActiveRecord we would have done this with a NOT NULL foreign key constraint at the database level.

It wasn't entirely obvious (to me) how to achieve this in an ActiveRecord model - you need *both* of the following two lines:

  validates_presence_of :category, :message => " must be specified" 
  validates_associated :category

Hope this helps someone.