Authorization in Rails 3.1 : CanCan, CanTango, declarative_authorization?

Question

I have looked at declarative_authorization, CanCan, and CanTango. They all are good in adding authorization to the application but I was wondering how does one add authorization to specific instance of a model i.e. a person can have a manage access in one project and only limited (read less than manage: limited update, etc) in another.

Could you please a better way? Apologies if my question sounds too trivial. It could be because I am new to RoR.

thanks, John