named_scope

http://ryandaigle.com/articles/2008/3/24/what-s-new-in-edge-rails-has-finder-functionality

class User < ActiveRecord::Base
 named_scope :registered, lambda { |time_ago| { :conditions => ['created_at > ?', time_ago] } }
end

class User < ActiveRecord::Base

 named_scope :inactive, :conditions => {:active => false} do
   def activate
     each { |i| i.update_attribute(:active, true) }
   end
 end
end
# Re-activate all inactive users
User.inactive.activate