Warning… please leave if you are not a geek. Consider leaving if you’re not a Ruby/Rails geek.

I am working on a project at work and one of the components is a set of sidewide announcements.

class Announcement < ActiveRecord::Base
  validates_presence_of :title, :body
 
  def self.active(time= DateTime.now)
      @active ||=  self.find(:all, :conditions => ["start_at <= ? and end_at >= ? and active=?", time, time, true], :order => "start_at desc")
  end
end

And all seemed to work on my development box. Then I noticed that when we went to production, that the announcements never seemed to go in and out of “active” state like they should. The application listed them as inactive, but they were always shown in the ‘announcements/_active’ partial. Hmm.

The problem is in using an instance variable inside a class method. The instance variable is now an instance of the Announcement Class object, not the Announcement object, so as soon as Rails was fired up, it would load up the active announcements and then that instance variable would stick around until the app got restarted.

But how can you properly cache items within class-level methods? Well, you can always use the Rails cache:

class Announcement < ActiveRecord::Base
  validates_presence_of :title, :body
 
  def self.active(time= DateTime.now)
    Rails.cache.fetch('Announcements::active', :expires => 30.minutes) do
      self.find(:all, :conditions => ["start_at <= ? and end_at >= ? and active=?", time, time, true], :order => "start_at desc")
    end
  end
end

Ahh… a rookie mistake, but one that I hope helps others.