Adding a test to demonstrate the bug

spec/acts_as_taggable_on/caching_spec.rb

@@ -99,6 +99,29 @@
     end
   end
 
+  describe 'Cache methods initialization on new models' do
+    before(:all) do
+      ActiveRecord::Schema.define do
+        create_table :cache_methods_injected_models do |t|
+          t.string :cached_tag_list
+        end
+      end
+      ActiveRecord::Base.connection.execute(
+        'INSERT INTO cache_methods_injected_models VALUES (NULL, \'ciao\')'
+      )
+      class CacheMethodsInjectedModel < ActiveRecord::Base
+        acts_as_taggable
+      end
+    end
+    after(:all) { Object.send(:remove_const, :CacheMethodsInjectedModel) }
+
+    it 'cached_tag_list_on? get injected correctly' do
+      expect do
+        CacheMethodsInjectedModel.first.tag_list
+      end.not_to raise_error
+    end
+  end
+
   describe 'CachingWithArray' do
     pending '#TODO'
   end