Friday, November 16, 2007

State change observer for ActiveRecord

When I started writing some code recently, I noticed that my controllers were getting fat. There was much to do, but there was a bunch of stuff in there that didn't have anything to do with actually carrying out the action--things like sending notifications. ActiveRecord already has observers to take action on certain callbacks. However, what I needed was to take actions on certain state transitions. Not seeing any immediate solutions in the Rails API, I decided to test myself and try writing one. I was bored too. So while I'm not sure if it was worth the time writing it, it certainly was kinda interesting. Here's what I came up with:

Just as a contrived example, let's say we are modeling the transmission of a car. It has three modes: "park", "reverse", "drive". We want to send a notification when a user tries to change it from "reverse" to "drive", but not when he tries to change it from "park" to "drive". If it didn't matter, and we just wanted to send notifications when the state changed to drive, we'd just use the observers that came with ActiveRecord. But since we do care where the state transition came from, here's what I came up with:

class CreateCarTransmission < ActiveRecord::Migration
def self.up
create_table :car_transmission do |t|
t.column :engine_id, :integer, :null => false
t.column :mode, :string, :null => false, :default => "park"

def self.down
drop_table :car_transmission

class CarTransmission < ActiveRecord::Base
include StateTransition::Observable
state_observable CarTransmissionNotifier, :state_name => :mode

So then for my notifier I have:

class CarTransmissionNotifier < StateTransition::Observer
def mode_from_drive_to_reverse(transmission)
# send out mail and flash lights about how this is bad.

And that's it. Whenever in the controller, I change the state from "reverse" to "drive", lights will flash and emails will be sent out condemning the action, and my controllers stay small and lean.

class CarController < ApplicationController
def dismantle
@car = Car.find(params[:id])
@car.update_attribute :mode, "reverse"
@car.update_attribute :mode, "drive"

So where's the magic? It took a bit of digging around. There were two major things I had to do. I had to insert observers during initialization and I had to override setting of attributes to include an update to notify observers.

ActiveRecord doesn't exactly allow you to override the constructor. I don't think I tried too hard to mess around with it. Looking on the web, I happened upon has_many :through again, where he has some good tips that helped me through Rail's rough edges. While I didn't exactly follow his advice, I did find out about the call back, :after_initialize. It must be something new, because I don't see it in the 2nd edition of the Rails book, and the current official API doesn't list it. Other Rails API manuals seem to be more comprehensive, like RailsBrain and Rails Manual.

Then overridding attributes has always been a bit of a mystery. I found a listing of the attribute update semantics, which was helpful to figure out what I was looking for, but it was false, in that you can't use the first one (article.attributes[:attr_name]=value) to set an attribute. Looking in the Rails code for 1.2.3, it shows that attributes is a read_only hash. But it's right that you should override the second one (article.attr_name=value), since update_attribute() and update_attributes() depends on it.

Again, it ends up that the function I was looking for wasn't found in the official API as a method, other than a short mention in the description of ActiveRecord under Overriding Attributes, which makes it harder to find. Ends up that we can use write_attribute().

So that's pretty much it. Using some standard meta-programming like how plugins do it, you wrap it up, and it's pretty simple:

require 'observer'

module StateTransition
module Observable
class StateNameNotFoundError < RuntimeError
def message
"option :state_name needs to be set to the name of an attribute"

def self.included(mod)

module ClassMethods

def state_observable(observer_class, options)
raise if options[:state_name].nil?
state_name = options[:state_name].to_s

include Object::Observable

define_method(:after_initialize) do

define_method("#{state_name}=") do |new_state|
old_state = read_attribute(state_name)
if old_state != new_state
write_attribute(state_name, new_state) # TODO yield the update method
notify_observers(self, state_name, old_state, new_state)



class Observer
def update(observable, state_name, old_state, new_state)
send("#{state_name}_from_#{old_state}_to_#{new_state}", observable)
rescue NoMethodError => e
# ignore any methods not found here


I had a difficult time figuring out how to define methods for an instance of a class. The only thing I came up with was to use define_method, or to include a module with instance methods in them. instance_eval() didn't work. The meta programming for ruby gets rather confusing when you're doing it inside a method--it seems hard to keep track of which context you're in.

So if you can make a use of this, great. If you think it's worth moving it into a plugin, let me know that too. If you know of a better way, by all means, let me know. tip!


  1. Anonymous12:31 PM

    +1 for plugin

  2. Anonymous3:22 PM

    Perhaps acts_as_state_machine would make your life easier?

  3. at acts_as_state_machine before, but it only calls the callbacks on arriving at the state, not during the transitions of the state. That's the only difference, really.

  4. Jenda6:52 AM

    Is there a way to notify the observers in the moment, when the state is actually stored in the database ? (I can change the state but not call the save method)