Change Event#thread_id to #transaction_id. Defaults to one "transaction" per thread but you can explicitly declare the start of a new one. This makes it possible for each request to have it own id.

activesupport/lib/active_support/notifications.rb

@@ -16,8 +16,8 @@ module ActiveSupport
   #
   #   @events = []
   #
-  #   ActiveSupport::Notifications.subscribe do |event|
-  #     @events << event
+  #   ActiveSupport::Notifications.subscribe do |*args|
+  #     @events << ActiveSupport::Notifications::Event.new(*args)
   #   end
   #
   #   ActiveSupport::Notifications.instrument(:render, :extra => :information) do
@@ -25,7 +25,6 @@ module ActiveSupport
   #   end
   #
   #   event = @events.first
-  #   event.class     #=> ActiveSupport::Notifications::Event
   #   event.name      #=> :render
   #   event.duration  #=> 10 (in miliseconds)
   #   event.result    #=> "Foo"
@@ -45,7 +44,7 @@ module ActiveSupport
     mattr_accessor :queue
 
     class << self
-      delegate :instrument, :transaction_id, :generate_id, :to => :instrumenter
+      delegate :instrument, :transaction_id, :transaction, :to => :instrumenter
 
       def instrumenter
         Thread.current[:notifications_instrumeter] ||= Instrumenter.new(publisher)
@@ -118,15 +117,15 @@ module ActiveSupport
     end
 
     class Event
-      attr_reader :name, :time, :end, :thread_id, :result, :payload
+      attr_reader :name, :time, :end, :transaction_id, :result, :payload
 
-      def initialize(name, start, ending, result, thread_id, payload)
-        @name      = name
-        @payload   = payload.dup
-        @time      = start
-        @thread_id = thread_id
-        @end       = ending
-        @result    = result
+      def initialize(name, start, ending, result, transaction_id, payload)
+        @name           = name
+        @payload        = payload.dup
+        @time           = start
+        @transaction_id = transaction_id
+        @end            = ending
+        @result         = result
       end
 
       def duration

activesupport/test/notifications_test.rb

@@ -124,6 +124,26 @@ class NotificationsMainTest < Test::Unit::TestCase
     assert_equal Hash[:payload => "notifications"], @events.last.payload
   end
 
+  def test_subscribed_in_a_transaction
+    @another = []
+
+    ActiveSupport::Notifications.subscribe("cache") do |*args|
+      @another << ActiveSupport::Notifications::Event.new(*args)
+    end
+
+    ActiveSupport::Notifications.instrument(:cache){ 1 }
+    ActiveSupport::Notifications.transaction do
+      ActiveSupport::Notifications.instrument(:cache){ 1 }
+    end
+    ActiveSupport::Notifications.instrument(:cache){ 1 }
+
+    sleep 0.1
+
+    before, during, after = @another.map {|e| e.transaction_id }
+    assert_equal before, after
+    assert_not_equal before, during
+  end
+
   def test_subscriber_with_pattern
     @another = []