singleton active record

hello,
I am trying to define a singleton active record on a rails 3.1 app.
What's missing please?

class Bitcoin < Currency
  include ActiveRecord::Singleton

end

When I run Bitcoin.instance
get this error:

NameError: uninitialized constant ActiveRecord::Singleton

Hi!

Your Currency class extends ActiveRecord::Base ?

Best Regards,

Everaldo

Found the answer. thanks. I had to install gem "acts_as_singleton"