diff --git a/activerecord/lib/active_record/associations/builder/collection_association.rb b/activerecord/lib/active_record/associations/builder/collection_association.rb
index 6cce519494..076fedc1a9 100644
--- a/activerecord/lib/active_record/associations/builder/collection_association.rb
+++ b/activerecord/lib/active_record/associations/builder/collection_association.rb
@@ -12,7 +12,7 @@ module ActiveRecord::Associations::Builder
                :after_add, :before_remove, :after_remove, :extend]
     end
 
-    attr_reader :block_extension, :extension_module
+    attr_reader :block_extension
 
     def initialize(*args, &extension)
       super(*args)
@@ -20,7 +20,7 @@ module ActiveRecord::Associations::Builder
     end
 
     def build
-      wrap_block_extension
+      define_extensions(model)
       reflection = super
       reflection
     end
@@ -30,7 +30,7 @@ module ActiveRecord::Associations::Builder
       CALLBACKS.each { |callback_name| define_callback(model, callback_name) }
     end
 
-    def wrap_block_extension
+    def define_extensions(model)
       if block_extension
         @extension_module = mod = Module.new(&block_extension)
         silence_warnings do
diff --git a/activerecord/test/cases/associations/extension_test.rb b/activerecord/test/cases/associations/extension_test.rb
index da767a2a7e..b1dacaefcc 100644
--- a/activerecord/test/cases/associations/extension_test.rb
+++ b/activerecord/test/cases/associations/extension_test.rb
@@ -59,9 +59,12 @@ class AssociationsExtensionsTest < ActiveRecord::TestCase
   end
 
   def test_extension_name
-    assert_equal 'DeveloperAssociationNameAssociationExtension', extension_name(Developer)
-    assert_equal 'MyApplication::Business::DeveloperAssociationNameAssociationExtension', extension_name(MyApplication::Business::Developer)
-    assert_equal 'MyApplication::Business::DeveloperAssociationNameAssociationExtension', extension_name(MyApplication::Business::Developer)
+    extend!(Developer)
+    extend!(MyApplication::Business::Developer)
+    extend!(MyApplication::Business::Developer)
+
+    assert Object.const_get 'DeveloperAssociationNameAssociationExtension'
+    assert MyApplication::Business.const_get 'DeveloperAssociationNameAssociationExtension'
   end
 
   def test_proxy_association_after_scoped
@@ -72,9 +75,8 @@ class AssociationsExtensionsTest < ActiveRecord::TestCase
 
   private
 
-    def extension_name(model)
+    def extend!(model)
       builder = ActiveRecord::Associations::Builder::HasMany.new(model, :association_name, nil, {}) { }
-      builder.send(:wrap_block_extension)
-      builder.extension_module.name
+      builder.define_extensions(model)
     end
 end