CanCan is an authorization solution for Ruby on Rails. This restricts what a given user is allowed to access throughout the application. It is completely decoupled from any role based implementation and focusses on keeping permission logic in a single location (the Ability class) so it is not duplicated across controllers, views, and database queries.
CanCan is provided as a gem. Simply include it in the environment.rb for Rails 2.3.
Or the Gemfile in Rails 3.
Alternatively it can be installed as a plugin.
script/plugin install git://github.com/ryanb/cancan.git
First, define a class called Ability in “models/ability.rb”. It should look something like this.
class Ability include CanCan::Ability def initialize(user) if user.admin? can :manage, :all else can :read, :all end end end
This is where all permissions will go. See the “Defining Abilities” section below for more information.
The current user's permissions can be accessed using the “can?” and “cannot?” methods in the view and controller.
<% if can? :update, @article %> <%= link_to "Edit", edit_article_path(@article) %> <% end %>
See Checking Abilities for more information
The “authorize!” method in the controller will raise an exception if the user is not able to perform the given action.
def show @article = Article.find(params[:id]) authorize! :read, @article end
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.
class ArticlesController < ApplicationController load_and_authorize_resource def show # @article is already loaded and authorized end end
See Authorizing Controller Actions for more information
If the user authorization fails, a CanCan::AccessDenied exception will be raised. You can catch this and modify its behavior in the ApplicationController.
class ApplicationController < ActionController::Base rescue_from CanCan::AccessDenied do |exception| flash[:error] = exception.message redirect_to root_url end end
See Exception Handling for more information.
As shown above, the Ability class is where all user permissions are defined. The user model is passed into the initialize method so the permissions can be modified based on any user attributes. CanCan makes no assumptions about how roles are handled in your application. See Role Based Authorization for an example.
The can method is used to define permissions and requires two arguments. The first one is the action you're setting the permission for, the second one is the class of object you're setting it on.
can :update, Article
You can pass an array for either of these parameters to match any one. In this case the user will have the ability to update or destroy both articles and comments.
can [:update, :destroy], [Article, Comment]
Use :manage to represent any action and :all to represent any class. Here are some examples.
can :manage, Article # has permissions to do anything to articles can :read, :all # has permission to read any model can :manage, :all # has permission to do anything to any model
You can pass a hash of conditions as the third argument to further restrict what the user is able to access. Here the user will only have permission to read active projects which he owns.
can :read, Project, :active => true, :user_id => user.id
See Defining Abilities with Hashes for more information.
Blocks can also be used if you need more control.
can :update, Project do |project| project && project.groups.include?(user.group) end
If the block returns true then the user has that :update ability for that project, otherwise he will be denied access. See Defining Abilities with Blocks for more information.
You will usually be working with four actions when defining and checking permissions: :read, :create, :update, :destroy. These aren't the same as the 7 RESTful actions in Rails. CanCan automatically adds some default aliases for mapping those actions.
alias_action :index, :show, :to => :read alias_action :new, :to => :create alias_action :edit, :to => :update
Notice the edit action is aliased to update. If the user is able to update a record he also has permission to edit it. You can define your own aliases in the Ability class
alias_action :update, :destroy, :to => :modify can :modify, Comment can? :update, Comment # => true
See Custom Actions for information on adding other actions.
In the controller index action you may want to fetch only the records which the user has permission to read. You can do this with the accessible_by scope.
@articles = Article.accessible_by(current_ability)
See Fetching Records for more information.