CanCan is an authorization library for Ruby on Rails which restricts what resources a given user is allowed to access. All permissions are defined in a single location (the +Ability+ class) and not duplicated across controllers, views, and database queries.
CanCan expects a +current_user+ method to exist. If you have not already, set up some authentication (such as Authlogic[http://github.com/binarylogic/authlogic] or Devise[http://github.com/plataformatec/devise]). See {Changing Defaults}[http://wiki.github.com/ryanb/cancan/changing-defaults] if you need different behavior.
Next create a class called +Ability+ in "models/ability.rb" or anywhere else in the load path. It should look similar to this.
The +current_user+ is passed in to this method which is where the abilities are defined. See {Defining Abilities}[http://wiki.github.com/ryanb/cancan/checking-abilities] for what can go here.
Setting this for every action can be tedious, therefore the +load_and_authorize_resource+ method is provided to automatically authorize all actions in a RESTful style resource controller. It will use a before filter to load the resource into an instance variable and authorize it for each action.
If the user authorization fails, a <tt>CanCan::AccessDenied</tt> exception will be raised. You can catch this and modify its behavior in the +ApplicationController+.
If you have any issues with CanCan which you cannot find the solution to in the documentation, please add an {issue on GitHub}[http://github.com/ryanb/cancan/issues] or fork the project and send a pull request.
To get the specs running you should call +bundle+ and then +rake+. Specs currently do not work in Ruby 1.9 due to the RR mocking framework. See the {spec/README}[https://github.com/ryanb/cancan/blob/master/spec/README.rdoc] for more information.
CanCan was inspired by declarative_authorization[http://github.com/stffn/declarative_authorization/] and aegis[http://github.com/makandra/aegis]. Also many thanks to the CanCan contributors[http://github.com/ryanb/cancan/contributors]. See the CHANGELOG[http://github.com/ryanb/cancan/blob/master/CHANGELOG.rdoc] for the full list.