In Ruby 1.9.3 (and I assume others, although I haven't checked), Module defines const_missing to be a public method. Virtus::ClassMethods however, redefines const_missing to be private. Ultimately, there are cases when the following exception occurs:
NoMethodError in MyController#show
private method `const_missing' called for My::Klass:Class
As a test, I simply moved the method definition until the public space of ClassMethods and everything works as expected.