assignable_values Show archive.org snapshot now lets you define a secondary default that is only used if the primary default value is not assignable:

class Song < ActiveRecord::Base
  assignable_values_for :year, :default => 1999, :secondary_default => lambda { Date.today.year } do
    (Date.today.year - 2) .. Date.today.year
  end
end

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If called in 2013 the code above will fall back to:

Song.new.year # => 2013

This is especially useful in authorization scenarios with Consul Show archive.org snapshot where you want powerful defaults for superusers, but don't want to make records invalid by default for users with lesser privileges.