Why isn't the composite_primary_keys gem for Rails working?

I've followed the instructions here, installing the composite_primary_keys gem via ``` sudo gem install composite_primary_keys ``` That worked fine. Now when I add the following to my model ``` set_primary_keys :user_id, :group_id ``` and I get ``` undefined method `set_primary_keys' for #<Class:0x1043bfe20> ``` Also, using multiple primary keys in a migration as described here has no effect. Any ideas why this might not be working and how to make it work? Note: I do not want a speech on why I should not be using composite keys--I have already made up my mind and just want to get this working. Thanks!