A Mixin To Add Attribute Change Tracking to Models

change_tracking.rb

require 'active_support/concern'

module Model
  ChangeEvent = Struct.new(:event, :condition)

  class ChangeTracker
    def initialize
      @attributes = {}
    end

    def track(attribute, event:, condition:)
      @attributes[attribute.to_s.freeze] = ChangeEvent.new(event, condition)
    end

    def process_changeset(client, changes)
      @attributes.keys.each do |attribute|
        old, new = changes[attribute]
        process(client, @attributes[attribute], old, new)
      end
    end

    private

    def process(client, change_event, old_value, new_value)
      if is_valid(change_event, old_value, new_value)
        # TrackingEvents.create!(...)
        puts "Trigger #{change_event.event} event for model attribute change from #{old_value} to #{new_value}"
      else
        puts "Did not trigger #{change_event.event} event for model attribute change from #{old_value} to #{new_value}, condition returned false."
      end
    end

    def is_valid(change_event, old_value, new_value)
      change_event.condition.call(old_value, new_value)
    end
  end

  module ChangeTracking
    extend ActiveSupport::Concern

    class_methods do
      def change_tracking_tracker
        @change_tracking_tracker ||= ChangeTracker.new
      end

      def track(attribute, event:, condition:)
        change_tracking_tracker.track(attribute, event: event, condition: condition)
      end
    end

    included do
      around_update :trigger_for_update

      def trigger_for_update
        raise 'Model must respond to "client" when implement ChangeTracking!'.freeze unless self.respond_to? :client

        puts "*" * 50
        changeset = changes
        puts "Triggering change event for attributes: #{changeset}"
        # Run update SQL...
        yield
        # Trigger change events if need be...
        self.class.change_tracking_tracker.process_changeset(client, changeset)
        puts "*" * 50
      end
    end
  end
end

An example of how this would be used:

ClientAcademicData.class_eval do
  include Model::ChangeTracking
  track :class_rank, event: :class_rank, condition: -> (old_value, new_value) { old_value != new_value }
end

cad = ClientAcademicData.last
cad.class_rank = 2
cad.save!

# **************************************************
# Triggering change event for attributes: {"class_rank"=>[50, 2], "mod_date"=>[Wed, 30 Sep 2015 15:37:39 CDT -05:00, Wed, 30 Sep 2015 15:38:24 CDT -05:00]}
# Trigger class_rank event for model attribute change from 50 to 2
# **************************************************

cad.class_rank = 2
cad.save!

# **************************************************
# Triggering change event for attributes: {"mod_date"=>[Wed, 30 Sep 2015 15:38:24 CDT -05:00, Wed, 30 Sep 2015 15:38:25 CDT -05:00]}
# Did not trigger class_rank event for model attribute change from  to , condition returned false.
# **************************************************

cad.class_rank = 50
cad.save!

# **************************************************
# Triggering change event for attributes: {"class_rank"=>[2, 50], "mod_date"=>[Wed, 30 Sep 2015 15:38:25 CDT -05:00, Wed, 30 Sep 2015 15:38:25 CDT -05:00]}
# Trigger class_rank event for model attribute change from 2 to 50
# **************************************************

• You can run this from a pry session once you've copied the above into app/concerns/models/change_tracking.rb.
• You usually would just mix this into the model class instead of reopening the class. I did that to easily spike it out / prove it works.
• Allows you to declaratively define the field to track, the name of the event it fires and finally a proc that takes the old and new values and returns true/false to indicate if the event should fire.