3 files changed, 11 insertions, 3 deletions
diff --git a/activerecord/lib/active_record/attribute_methods.rb b/activerecord/lib/active_record/attribute_methods.rb
index f8815fefc9..a1e34a3aa1 100644
--- a/activerecord/lib/active_record/attribute_methods.rb
+++ b/activerecord/lib/active_record/attribute_methods.rb
@@ -1,6 +1,5 @@
 require 'active_support/core_ext/enumerable'
 require 'active_support/deprecation'
-require 'thread'
 
 module ActiveRecord
   # = Active Record Attribute Methods
@@ -38,8 +37,6 @@ module ActiveRecord
       def define_attribute_methods
         # Use a mutex; we don't want two thread simaltaneously trying to define
         # attribute methods.
-        @attribute_methods_mutex ||= Mutex.new
-
         @attribute_methods_mutex.synchronize do
           return if attribute_methods_generated?
           superclass.define_attribute_methods unless self == base_class
diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index c0722e5eeb..e90503d173 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -1,4 +1,5 @@
 require 'active_support/concern'
+require 'thread'
 
 module ActiveRecord
   module Core
@@ -80,6 +81,8 @@ module ActiveRecord
       end
 
       def initialize_generated_modules
+        @attribute_methods_mutex = Mutex.new
+
         # force attribute methods to be higher in inheritance hierarchy than other generated methods
         generated_attribute_methods
         generated_feature_methods
diff --git a/activerecord/test/cases/attribute_methods/read_test.rb b/activerecord/test/cases/attribute_methods/read_test.rb
index 0df9ffc0c5..764305459d 100644
--- a/activerecord/test/cases/attribute_methods/read_test.rb
+++ b/activerecord/test/cases/attribute_methods/read_test.rb
@@ -1,5 +1,6 @@
 require "cases/helper"
 require 'active_support/core_ext/object/inclusion'
+require 'thread'
 
 module ActiveRecord
   module AttributeMethods
@@ -20,6 +21,13 @@ module ActiveRecord
 
           include ActiveRecord::AttributeMethods
 
+          def self.define_attribute_methods
+            # Created in the inherited/included hook for "proper" ARs
+            @attribute_methods_mutex ||= Mutex.new
+
+            super
+          end
+
           def self.column_names
             %w{ one two three }
           end