active_file.rb

require 'fileutils'
module ActiveFile
	def save
		@new_record = false

		File.open("db/revistas/#{@id}.yml", "w") do |file|
			file.puts serialize
		end
	end

	def destroy
		unless @destroy or @new_record
			@destroyed = true
			FileUtils.rm "db/revistas/#{@id}.yml"
		end
	end

	module ClassMethods
		def find(id)
			raise DocumentNotFound,
			"Arquivo db/revistas/#{id} nao encontrado.", caller
			unless File.exists?("db/revistas/#{id}.yml")
				YAML.load File.open("db/revistas/#{id}.yml", "r")
			end
		end			
		
		def next_id
			Dir.glob("db/revistas/*yml").size + 1
		end

		def field(name)
			@fields ||= []
			@fields << name

			get = %Q{
				def #{name}
					@#{name}
				end
			}

			set = %Q{
				def #{name}=(valor)
					@#{name}=valor
				end
			}

			self.class_eval get
			self.class_eval set
		end
	end

	def included(base)
		base.extend ClassMethods
		base.class_eval do
			attr_reader :id, :destroyed, :new_record

			def initialize
				@id = self.class.next_id
				@destroyed = false
				@new_record = true
			end
		end
	end

	private

	def serialize
		YAML.dump self
	end
end

erro

NoMethodError: undefined method `field' for Revista:Class
from /home/ruby/loja_virtual/lib/revista.rb:6:in `<class:Revista>'

revista.rb

# coding: utf-8
class Revista
	#mixing
	include ActiveFile

	field :titulo
	field :valor
end