State change observer for ActiveRecord in Rails

iamwil (wilhelm) — Fri, 16 Nov 2007 20:07:04 GMT

This is something to observe state changes as indicated by an attribute in an ActiveRecord Object in Rails. Normally, if you just need to observe an object and do things based on what state it's in, use the Observer provided in Rails. You can then use callbacks to do whatever you need to in the observer. However, if you want to do something upon a particular state transition, then you can try the stuff below:

Create a model like this. Here, the attribute mode keeps track of the state.



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"

    end

  end



  def self.down

    drop_table :car_transmission

  end

end

In the model, you include the state transition observable class, and call state_observable.



class CarTransmission < ActiveRecord::Base

  include StateTransition::Observable

  state_observable CarTransmissionNotifier, :state_name => :mode

end

Then you define a notifier class that has different callbacks based on the transitions. It follows the pattern, "#{state_name}_from_#{state you're coming from}_to_#{state you're going to}"



class CarTransmissionNotifier < StateTransition::Observer

  def mode_from_drive_to_reverse(transmission)

    # send out mail and flash lights about how this is bad.

  end

end

So whenever you change the state from "drive" to "reverse" of a car transmission model in the controller, the notifier will fire up.

To use it, put the following code in a file called 'state_transition.rb', put it in your lib/ directory in rails.



require 'observer'



module StateTransition

  module Observable

    class StateNameNotFoundError < RuntimeError

      def message

        "option :state_name needs to be set to the name of an attribute"

      end

    end



    def self.included(mod)

      mod.extend(ClassMethods)

    end



    module ClassMethods



      def state_observable(observer_class, options)

        raise StateNameNotFoundError.new if options[:state_name].nil?

        state_name = options[:state_name].to_s

        

        include Object::Observable



        define_method(:after_initialize) do 

          add_observer(observer_class.new)

        end



        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

            changed

            notify_observers(self, state_name, old_state, new_state)

          end

        end

      end



    end

    

  end



  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

    end  

  end



end

more details here

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State change observer for ActiveRecord in Rails