Trouble with package structure

Hi,

I'm writing a library class for my Rails application stored in
RAILS_ROOT/lib/wrap. My class looks like this:

class Wrap::ModuleManager
   # Code...
end

## Test this class
mmanager = Wrap::ModuleManager.new

I'd like to test this class without having to run it through Rails,
but when I try and run it using:

$ ruby module_manager.rb

I get the following error:

module_manager.rb:5: uninitialized constant Wrap (NameError)

I must be doing something really obvious wrong and this is probably
due to not enough general Ruby experience. Could anyone help?

Thanks in advance,
Rhys