defning extensions happens once, no need to cache

activerecord/lib/active_record/associations/builder/collection_association.rb

@@ -32,7 +32,9 @@ def define_callbacks(model, reflection)
 
     def define_extensions(model)
       if block_extension
-        @extension_module = mod = Module.new(&block_extension)
+        mod = Module.new(&block_extension)
+        extension_module_name = "#{model.name.demodulize}#{name.to_s.camelize}AssociationExtension"
+
         silence_warnings do
           model.parent.const_set(extension_module_name, mod)
         end
@@ -47,10 +49,6 @@ def define_extensions(model)
       end
     end
 
-    def extension_module_name
-      @extension_module_name ||= "#{model.name.demodulize}#{name.to_s.camelize}AssociationExtension"
-    end
-
     def define_callback(model, callback_name)
       full_callback_name = "#{callback_name}_for_#{name}"