ActiveRecord state machine

stateful.rb

module Instructify

  # Mark an ActiveRecord model as having a state machine. Requires a
  # string attribute called `state`. To set the default state for a
  # class, set a default column value for `state` in the database.
  #
  # Use Symbols for all keys and values in state definitions.

  module Stateful
    ANY = Object.new

    def self.included model
      model.validates :state, presence: true

      model.extend Macro

      # Default states that are valid on all models that include Stateful
      model.stateful do
        state :active
        state :inactive
        state :deleted

        on :activate do
          move :inactive => :active
        end

        on :deactivate do
          move :active => :inactive
        end

        on :delete do
          move any => :deleted
        end
      end
    end

    module Macro
      def machine
        @machine ||= Machine.new self
      end

      def stateful &block
        machine.instance_eval(&block)
      end

      def states
        machine.states
      end
    end

    class Context < Struct.new :instance, :event, :src, :dest, :args
      def trigger hooks
        hooks.values_at(dest, ANY).compact.flatten.each do |hook|
          instance.instance_exec self, &hook
        end
      end
    end

    class Event
      def initialize model, name
        @moves = {}

        model.send :define_method, "#{name}!" do |*args|
          model.machine.fire self, name, *args
        end
      end

      def any
        ANY
      end

      def dest current
        @moves[current.to_sym] || @moves[any]
      end

      def move pair
        Array(pair.keys.first).each do |s|
          @moves[s] = pair.values.first
        end
      end
    end

    class Machine
      def initialize model
        @model      = model
        @states     = []
        @entered    = Hash.new { |h, k| h[k] = [] }
        @entering   = Hash.new { |h, k| h[k] = [] }
        @events     = Hash.new { |h, k| h[k] = Event.new @model, k }
        @persisted  = Hash.new { |h, k| h[k] = [] }
      end

      def entered *names, &block
        munge(names).each { |n| @entered[n] << block }
      end

      def entering *names, &block
        munge(names).each { |n| @entering[n] << block }
      end

      def fire instance, name, *args
        raise "No [#{name}] event." unless @events.include? name

        src  = instance.state
        dest = @events[name].dest src

        unless dest
          raise "Can't [#{name}] while [#{instance.state}]: #{instance}"
        end

        ctx = Context.new instance, name, src.to_sym, dest, args

        ActiveRecord::Base.transaction do
          ctx.trigger @entering

          instance.state = ctx.dest.to_s
          ctx.trigger @entered

          instance["#{ctx.dest.to_s}_at"] = DateTime.now if instance.has_attribute? "#{ctx.dest.to_s}_at"

          instance.save!
        end

        ctx.trigger @persisted

        instance
      end

      def on *names, &block
        names.flatten.each { |n| @events[n].instance_eval(&block) }
      end

      def persisted *names, &block
        munge(names).each { |n| @persisted[n] << block}
      end

      def state name
        name = name.to_s

        @states.push(name).uniq!

        unless @model.respond_to? name
          @model.scope name, -> { @model.where(state: name) }
          @model.scope "not_#{name}", -> { @model.where("state != '#{name}'") }
          @model.send(:define_method, "#{name}?") { name == (state && state.to_s) }
        end
      end

      def states
        @states
      end

      private

      def munge names
        names.flatten!
        names.empty? ? [ANY] : names
      end
    end
  end
end