Dirty state tracking for previous changes

previously_dirty.rb

module PreviouslyDirty
  extend ActiveSupport::Concern
  include ActiveModel::AttributeMethods

  included do
    attribute_method_suffix '_previously_changed?', '_previously_was'
  end

  # Handle <tt>*_previously_changed?</tt> for +method_missing+.
  def attribute_previously_changed?(attr, options = {}) #:nodoc:
    result = previous_changes.include?(attr)
    result &&= options[:to] == previous_changes[attr][1] if options.key?(:to)
    result &&= options[:from] == previous_changes[attr][0] if options.key?(:from)
    result
  end

  # Handle <tt>*_was</tt> for +method_missing+.
  def attribute_previously_was(attr) # :nodoc:
    attribute_previously_changed?(attr) ? previous_changes[attr][0] : __send__(attr)
  end
end

Rails has some great dirty state tracking methods but this information is cleared after the commit. This prevents you doing things like:

user = 'new_email@example.com'
user.save!
send_some_email if user.email_changed? # this'll never execute

but all the changes from the last commit are stored in the previous_changes hash so this gist adds the attribute_previously_changed? and attribute_previously_was methods following the same pattern as the regular change methods

user.email = 'new_email@example.com'
user.save!
send_some_email if user.email_previously_changed?  #=> now executes
old_email = user.email_previously_was                         #=> the old email address

I find this very handy for conditional after_commit callbacks or, even better, for avoiding callbacks altogether and putting that logic in the controller (on the basis that callbacks can be a complete PITA)