state_machine.rb

module SimpleStateMachine
  extend ActiveSupport::Concern

  # instance methods
  included do
    def current_step
      # model that includes this module must have a field called 'current_step'
      super || steps.first
    end

    def next_step
      (self.current_step = steps[steps.index(current_step)+1]) unless last_step?
    end

    def previous_step
      (self.current_step = steps[steps.index(current_step)-1]) unless first_step?
    end

    def first_step?
      current_step == steps.first
    end

    def last_step?
      current_step == steps.last
    end
  end # instance methods

  module ClassMethods
    # Define a simple linear state machine.
    #
    # steps - Array of Strings representing steps (order matters).
    #
    # Examples
    #
    #   has_linear_steps %w[step1 step2 stepN]
    #
    # Returns self
    def has_linear_steps(steps)
      define_method :steps do
        steps
      end
    end
  end # class methods
end