Add 'revision' attribute to Conference, managed by an observer

app/models/revision_observer.rb

@@ -0,0 +1,26 @@
+#
+# suseconferenceclient relies on a 'revision' attribute for caching and
+# doing some calculations.
+#
+# It should be incremented after any change in the conference or in any
+# associated models
+#
+# This observer updates the revision column in a non-intrusive way,
+# preventing validations, callbacks or exceptions to be triggered
+#
+# Relying on paper_trail could also be an option, but a 'revision' column
+# in table 'conferences' looks like a more simple and straightforward solution
+#
+class RevisionObserver < ActiveRecord::Observer
+  observe :conference, :event, :room, :social_event, :track
+
+  def after_save(model)
+    begin
+      conference = model.kind_of?(Conference) ? model : model.conference
+      conference.reload.increment(:revision)
+      conference.update_column(:revision, conference.revision)
+    rescue
+      nil
+    end
+  end
+end

config/application.rb

@@ -25,6 +25,7 @@ module Osem
 
     # Activate observers that should always be running.
     # config.active_record.observers = :cacher, :garbage_collector, :forum_observer
+    config.active_record.observers = :revision_observer
 
     # Set Time.zone default to the specified zone and make Active Record auto-convert to this zone.
     # Run "rake -D time" for a list of tasks for finding time zone names. Default is UTC.

db/migrate/20130705055128_add_revision_to_conference.rb

@@ -0,0 +1,5 @@
+class AddRevisionToConference < ActiveRecord::Migration
+  def change
+    add_column :conferences, :revision, :integer
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended to check this file into your version control system.
 
-ActiveRecord::Schema.define(:version => 20130626095459) do
+ActiveRecord::Schema.define(:version => 20130705055128) do
 
   create_table "call_for_papers", :force => true do |t|
     t.date     "start_date",    :null => false
@@ -62,6 +62,7 @@ ActiveRecord::Schema.define(:version => 20130626095459) do
     t.datetime "logo_updated_at"
     t.boolean  "use_dietary_choices",     :default => false
     t.boolean  "use_supporter_levels",    :default => false
+    t.integer  "revision"
   end
 
   create_table "dietary_choices", :force => true do |t|