Automatically create scopes for all available state_machine states in Rails 3.1

contact.rb

class Contact < ActiveRecord::Base
  
  state_machine :state, :initial => :imported do
    state :imported
    state :contacted
    state :verified
    state :prospect
    state :deleted
  end

  include StateMachineScopes
  state_machine_scopes :state

end

state_machine_scopes.rb

module StateMachineScopes
  class MethodAlreadyDefinedError < StandardError; end
  class StateMachineNotFoundError < StandardError; end

  module ClassMethods
    def all_states(machine = :state)
      state_machines[machine.to_sym].states.keys.sort
    end

    def state_machine_scopes(*which)
      options = which.last.is_a?(Hash) ? which.pop : {}
      prefix = options.delete(:prefix)

      if which.any?
        which.each do |machine_name|
          define_scopes_for(machine_name, prefix)
        end
      else
        state_machines.keys.each do |machine_name|
          define_scopes_for(machine_name, prefix)
        end
      end
    end

  private

    def define_scopes_for(machine_name, prefix)
      state_machines[machine_name.to_sym].states.keys.each do |state|
        scope_name = [prefix, machine_name, state].compact.join('_')
        if self.respond_to? scope_name
          raise MethodAlreadyDefinedError.new("Already defined method called \"#{scope_name}\"")
        end
        self.class_eval do
          scope :"#{scope_name}", where(:state => state)
        end
      end
    end

  end

  def self.included(klass)
    if klass.respond_to?(:state_machines)
      klass.extend(ClassMethods)
    else
      raise StateMachineNotFoundError.new("\"#{klass.name}\" does not seem to have any state machines.")
    end
  end

end