6 files changed, 49 insertions, 9 deletions
diff --git a/actionpack/lib/action_dispatch/http/response.rb b/actionpack/lib/action_dispatch/http/response.rb
index 06e936cdb0..60a2cccdc5 100644
--- a/actionpack/lib/action_dispatch/http/response.rb
+++ b/actionpack/lib/action_dispatch/http/response.rb
@@ -55,6 +55,7 @@ module ActionDispatch # :nodoc:
     CONTENT_TYPE = "Content-Type".freeze
     SET_COOKIE   = "Set-Cookie".freeze
     LOCATION     = "Location".freeze
+    NO_CONTENT_CODES = [204, 304]
 
     cattr_accessor(:default_charset) { "utf-8" }
     cattr_accessor(:default_headers)
@@ -289,7 +290,7 @@ module ActionDispatch # :nodoc:
 
       header[SET_COOKIE] = header[SET_COOKIE].join("\n") if header[SET_COOKIE].respond_to?(:join)
 
-      if [204, 304].include?(@status)
+      if NO_CONTENT_CODES.include?(@status)
         header.delete CONTENT_TYPE
         [status, header, []]
       else
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index ad766d3267..d47a23b8f5 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,14 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Fix for a regression bug in which counter cache columns were not being updated
+    when record was pushed into a has_many association. For example:
+
+        Post.first.comments << Comment.create
+
+    Fixes #3891.
+
+    *Matthew Robertson*
+
 *   If a model was instantiated from the database using `select`, `respond_to?`
     returns false for non-selected attributes. For example:
 
diff --git a/activerecord/lib/active_record/associations/builder/belongs_to.rb b/activerecord/lib/active_record/associations/builder/belongs_to.rb
index 3ba6a71366..092230b2f7 100644
--- a/activerecord/lib/active_record/associations/builder/belongs_to.rb
+++ b/activerecord/lib/active_record/associations/builder/belongs_to.rb
@@ -25,9 +25,10 @@ module ActiveRecord::Associations::Builder
 
       mixin.class_eval <<-CODE, __FILE__, __LINE__ + 1
         def belongs_to_counter_cache_after_create_for_#{name}
-          record = #{name}
-          record.class.increment_counter(:#{cache_column}, record.id) unless record.nil?
-          @_after_create_counter_called = true
+          if record = #{name}
+            record.class.increment_counter(:#{cache_column}, record.id)
+            @_after_create_counter_called = true
+          end
         end
 
         def belongs_to_counter_cache_before_destroy_for_#{name}
diff --git a/activerecord/test/cases/associations/has_many_associations_test.rb b/activerecord/test/cases/associations/has_many_associations_test.rb
index 781b87741d..d85570236f 100644
--- a/activerecord/test/cases/associations/has_many_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_associations_test.rb
@@ -755,6 +755,15 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
     assert_equal topic.replies.to_a.size, topic.replies_count
   end
 
+  def test_pushing_association_updates_counter_cache
+    topic = Topic.order("id ASC").first
+    reply = Reply.create!
+
+    assert_difference "topic.reload.replies_count", 1 do
+      topic.replies << reply
+    end
+  end
+
   def test_deleting_updates_counter_cache_without_dependent_option
     post = posts(:welcome)
 
diff --git a/activerecord/test/cases/dirty_test.rb b/activerecord/test/cases/dirty_test.rb
index 7b2034dadf..36b87033ae 100644
--- a/activerecord/test/cases/dirty_test.rb
+++ b/activerecord/test/cases/dirty_test.rb
@@ -213,9 +213,11 @@ class DirtyTest < ActiveRecord::TestCase
       topic = target.create
       assert_nil topic.written_on
 
-      topic.written_on = ""
-      assert_nil topic.written_on
-      assert !topic.written_on_changed?
+      ["", nil].each do |value|
+        topic.written_on = value
+        assert_nil topic.written_on
+        assert !topic.written_on_changed?
+      end
     end
   end
 
diff --git a/activesupport/lib/active_support/subscriber.rb b/activesupport/lib/active_support/subscriber.rb
index ab76cf5c85..34c6f900c1 100644
--- a/activesupport/lib/active_support/subscriber.rb
+++ b/activesupport/lib/active_support/subscriber.rb
@@ -1,3 +1,5 @@
+require 'active_support/per_thread_registry'
+
 module ActiveSupport
   # ActiveSupport::Subscriber is an object set to consume
   # ActiveSupport::Notifications. The subscriber dispatches notifications to
@@ -68,8 +70,24 @@ module ActiveSupport
 
     private
 
-    def event_stack
-      Thread.current[@queue_key] ||= []
+      def event_stack
+        SubscriberQueueRegistry.get_queue(@queue_key)
+      end
+  end
+
+  # This is a registry for all the event stacks kept for subscribers.
+  #
+  # See the documentation of <tt>ActiveSupport::PerThreadRegistry</tt>
+  # for further details.
+  class SubscriberQueueRegistry # :nodoc:
+    extend PerThreadRegistry
+
+    def initialize
+      @registry = {}
+    end
+
+    def get_queue(queue_key)
+      @registry[queue_key] ||= []
     end
   end
 end