make UserActionObserver work (Rails bug #4087)

Monkeypatch ActiveRecord::Observer to always return true after
`notify_observers` in "_notify_observers_for_#{method}" callbacks,
making it again possible to observe before_* events without these
methods halting the callbacks chain all the time.

app/models/user_action_observer.rb

@@ -4,10 +4,10 @@ class UserActionObserver < ActiveRecord::Observer
   cattr_accessor :current_user
 
   def before_create(model)
-    model.created_by = @@current_user
+    model.created_by = current_user
   end
 
   def before_update(model)
-    model.updated_by = @@current_user
+    model.updated_by = current_user
   end
 end

config/initializers/active_record_extensions.rb

@@ -1,6 +1,6 @@
 require 'active_record'
 
-class ActiveRecord::Base
+ActiveRecord::Base.class_eval do
   def self.object_id_attr(symbol, klass)
     module_eval %{
       def #{symbol}
@@ -16,3 +16,23 @@ def #{symbol}
     }
   end
 end
+
+ActiveRecord::Observer.class_eval do
+  protected
+  alias_method :original_add_observer!, :add_observer!
+
+  # monkeypatch to fix observers for before_* callbacks
+  def add_observer!(klass)
+    super
+
+    # Check if a notifier callback was already added to the given class. If
+    # it was not, add it.
+    self.class.observed_methods.each do |method|
+      callback = :"_notify_observers_for_#{method}"
+      if (klass.instance_methods & [callback, callback.to_s]).empty?
+        klass.class_eval "def #{callback}; notify_observers(:#{method}); true; end"
+        klass.send(method, callback)
+      end
+    end
+  end
+end