I often come across this particular use case and it’s probably easiest to explain with an example. Take an ActiveRecord model called Book which represents a book in Amazon’s catalogue. In addition to books which are already published, we collect information about books which are going to be released at some point in the future. We store the publication date in the field published_at.

More often than not, though, we don’t really want to know when a book was (or will be) published, we just want to know whether it has been published or not. So I wind up defining a couple of extra methods:

class Book < ActiveRecord::Base
  def published?
    !published_at.blank?
  end

  def published=(value)
    # Note the automatic time zone conversion magic in edge rails!
    self.published_at = value ? Time.now : nil
  end
end

All well and good, but I wind up doing that quite a few times across all the projects I have. Time for an extraction! And what’s good about this extraction? Well, it expresses what I am actually trying to do without boring you with the details of how I do it. So how about:

class Book < ActiveRecord::Base
  timestamped_boolean :published_at
end

Now we get the two methods defined automatically. And, as I say, we have a declarative way of saying what we’re providing without having to have extra documentation.

I’ve pulled this abstraction into a tiny wee plugin which is now up on GitHub. It’s called timestamped_booleans. Feel free to grab it, fork it, play around. I’ve got a couple of changes I should do and push in the next few hours. The first you’ll already have noticed if you’re reading carefully; what if the date it is published_at is in the future? It’s not published then, is it? So I’ll need to modify the question method to also check that the attribute’s date is not in the future.

Next up after that is to automatically declare associated named scopes for edge Rails because I also wind up doing the equivalent as class methods a lot:

class Book < ActiveRecord::Base
  def self.find_published(*args)
    with_published do
      find(*args)
    end
  end

  private
  def with_published
    with_scope :find => :conditions => { ['published_at IS NOT ?'], nil} do
      yield
    end
  end
end

which it wouldn’t hurt to tidy up a bit.