Using pluginaweek's state_machine, can I reference the activerecord object during an event?

Question

I'm trying to implement a "suspend" event that transitions the object to the :suspended state. But I need to be able to "unsuspend", and return to the previous state. I added a previous_state field to the model, but I can't see how to access it inside an event block.

This is the basic logic I'm trying to implement:

event :suspend do
  owner.previous_state = self.state
  transition [:new, :old] => :suspended
end

event :unsuspend do
  transition :suspended => owner.previous_state.to_sym
  owner.previous_state = nil
end

The state_machine docs haven't been very helpful, and I can't find examples online. Sometimes it's tough to know how to describe something to google :)

Answer 1

The author of state_machine has also provided an alternate solution here: https://groups.google.com/d/msg/pluginaweek-talk/LL9VJdL_x9c/vP1qv6br734J

To wit:

Another possible solution is to be a little creative with how the state machine works. There are plenty of hooks in ORMs like ActiveRecord that give us the ability to set the state at any stage in the process. Consider the following:

class Vehicle < ActiveRecord::Base
  before_validation do |vehicle|
    # Set the actual value based on the previous state if we've just restored
    vehicle.state = vehicle.previous_state if vehicle.restored?
  end

  state_machine :initial => :parked do
    event :ignite do
      transition :parked => :idling
    end

    event :restore do
      transition any => :restored
    end

    state :parked do
      validates_presence_of :name
    end
  end

  # Look up previous state here...
  def previous_state
    'parked'
  end
end

In this example, a new state, restored, is introduced even though it never actually gets persisted in the database. We instead provide a before_validation hook that rewrites the state based on the previous state. You can see the results below:

v = Vehicle.new(:name => 'test')  # => #<Vehicle id: nil, name: "test", state: "parked">
v.save                            # => true
v.name = nil                      # => nil
v.ignite                          # => true
v                                 # => #<Vehicle id: 1, name: nil, state: "idling">
v.restore                         # => false
v.errors                          # => #<OrderedHash {:name=>["can't be blank"]}>
v.state                           # => "idling"
v.name = 'test'                   # => "test"
v.restore                         # => true
v                                 # => #<Vehicle id: 1, name: "test", state: "parked">
v.parked?                         # => true

This should require one less database hit as it occurs before validation. In my case the complete picture looks like this:

module Interpreting::Status

  extend ActiveSupport::Concern

  included do

    before_validation :restore_previous_state, if: :interpreter_cancelled?

    state_machine :state, :initial => :ordered do

      before_transition :to => :interpreter_booked, :do => :set_previous_state

      state :ordered

      state :confirmed

      state :interpreter_booked

      state :interpreter_cancelled # Transient status
    end

  end


protected

  def set_previous_state
    self.previous_state = self.state
  end

  def restore_previous_state
    self.state = self.previous_state
  end

end