gistfile1.rb

module FieldTracker
  def self.included(cls)
    cls.extend(ClassMethods)
    cls.before_save :save_changes
    cls.after_save  :check_changes
  end

  def save_changes
    @changes ||= []
    hash = {}
    changes.each do |field,(_,value)|
      hash[field.to_sym] = value
    end
    @changes.push(hash)
  end

  def check_changes
    changes = @changes.pop
    return if changes.empty?

    self.class.tracked_fields.each do |fields, block|
      hash = changes.select {|k,v| fields.include?(k)}
      block.call(self, hash) unless hash.empty?
    end
  end

  module ClassMethods
    def observe_fields(*fields, &block)
      raise 'no fields given' if fields.empty?
      tracked_fields[fields.map{|f| f.to_sym}] = block
    end
    alias :observe_field :observe_fields

    def tracked_fields
      @tracked_fields ||= {}
    end
  end
end