Gotcha: composite_primary_keys gem

Dr. Nic's composite_primary_keys gem is incredibly helpful if you're writing rails code against a legacy database, or in any other situation where you can't or won't follow the rails convention of having a single field as your model's primary key.
After doing the usual;

sudo gem install composite_primary_keys

...and requiring it in you environment.rb file, you can define a model like this;

class Membership < ActiveRecord::Base
set_primary_keys :user_id, :group_id
...
end

Brilliant.
But, there is one subtlety you need to be aware of. If your model mixes in any modules, you might end up writing something like this;

class Membership < ActiveRecord::Base
include MyAwesomeModule
set_primary_keys :user_id, :group_id
...
end

Looks fine, doesn't it? Unfortunately, your tests will now break with lots of errors like this;

ActiveRecord::StatementInvalid in 'Membership should foobar'
Mysql::Error: Column count doesn't match value count at row 1: INSERT INTO memberships (`user_id`, `group_id`, ... , id) VALUES (...whatever...)
/Users/david/myproj/vendor/composite_primary_keys-0.9.90/lib/composite_primary_keys/base.rb:106:in `create_without_callbacks'

See that last id, just before VALUES? It shouldn't be there. Something weird is going on, because composite_primary_keys doesn't seem to be doing its thing.
The solution is to make sure the call to "set_primary_keys" is the first thing executed in your model;

class Membership < ActiveRecord::Base
set_primary_keys :user_id, :group_id
include MyAwesomeModule
...
end

Remember this, and everything works fine. Forget it, and you'll have lots of fun with the debugger.
If I were braver, smarter and kinder, I would dive headfirst into the code and try to fix it. But, I've got work to do.