A simple ActiveRecord state machine for Mongoid

simple_state_machine.rb

module SimpleStateMachine
  # Usage:
  #
  # First, extend your model of choise with this module
  #
  # extend SimpleStateMachine
  #
  # Then you can call the method states() and pass all the states
  # that your model supports. E.g.:
  #
  # states('ready', 'finished')
  #
  # This will create the following methods:
  #
  # Model.ready?, Model.ready!
  # Model.finished, Model.finished!
  #
  # And define the following fields:
  #
  # Model.status (string)
  # Model.ready_at (time)
  # Model.finished_at (time)
  #
  # If you don't want to create the status field, for example
  # if it's already being used for something else, you can pass
  # false as the first parameter to the states() method, like this:
  #
  # states(false, 'ready', 'finished')
  #
  # The default is 'created' or the first passed in status if 'created' is not available
  #
  def states(*states)
    use_status_field = true
    if states.first.is_a?(Boolean)
      use_status_field = states.shift
    end

    if use_status_field
      default_status = 'created'
      default_status = states.first unless default_status.in?(states)
      field :status, type: String, default: default_status
      validates :status, presence: true, inclusion: { in: states }
    end

    states.each { |s| state s, use_status_field } # Pass the use_status_field along.

    define_method :status_at do
      if read_attribute("#{status}_at")
        read_attribute("#{status}_at")
      else
        read_attribute("updated_at")
      end
    end
  end

  def state(state, use_status_field, &_block)
    field "#{state}_at", type: Time

    scope "not_#{state}", -> { self.or({ :"#{state}_at".gte => Time.now }, { :"#{state}_at" => nil }) }
    scope state, -> { where(:"#{state}_at".lte => Time.now) }

    define_method "#{state}?" do
      !!(read_attribute("#{state}_at") && read_attribute("#{state}_at") <= Time.now)
    end

    define_method "#{state}" do
      write_attribute "#{state}_at", Time.now
      if use_status_field
        write_attribute :status, state
      end
    end

    define_method "#{state}!" do
      return if send("#{state}?")

      send(state) # Call the method w/o the bang.
      save!

      yield if block_given?
    end
  end
end