This is a simple authorization solution for Ruby on Rails to restrict what a given user is allowed to access in the application. This is completely decoupled from any role based implementation allowing you to define user roles the way you want. All permissions are stored in a single location for convenience.
This assumes you already have authentication (such as Authlogic) which provides a current_user model.
You can set it up as a gem in your environment.rb file.
config.gem "cancan", :source => "http://gemcutter.org"
And then install the gem.
sudo rake gems:install
Alternatively you can install it as a Rails plugin.
script/plugin install git://github.com/ryanb/cancan.git
First, define a class called Ability in “models/ability.rb”.
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.
You can access the current permissions at any point using the “can?” and “cannot?” methods in the view.
<% if can? :update, @article %> <%= link_to "Edit", edit_article_path(@article) %> <% end %>
You can also use these methods in a controller along with the “unauthorized!” method to restrict access.
def show @article = Article.find(params[:id]) unauthorized! if cannot? :read, @article end
Setting this for every action can be tedious, therefore a before filter is also provided to automatically authorize all actions in a RESTful style resource controller.
class ArticlesController < ApplicationController before_filter :load_and_authorize_resource def show # @article is already loaded end end
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, :with => :access_denied protected def access_denied flash[:error] = "Sorry, you are not allowed to access that page." redirect_to root_url end end
As shown above, the Ability class is where all user permissions are defined. The user model is passed into the initialize method so you are free to modify the permissions based on the user's attributes. This way CanCan is completely decoupled with how you choose to handle roles.
The “can” method accepts 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.
can [:update, :destroy], [Article, Comment]
In this case the user has the ability to update or destroy both articles and comments.
You can pass a block to provide logic based on the article's attributes.
can :update, Article do |article| article && article.user == user end
If the block returns true then the user has that :update ability for that article, otherwise he will be denied access. It's possible for the passed in model to be nil if one isn't specified, so be sure to take that into consideration.
You can pass :all to reference every type of object. In this case the object type will be passed into the block as well (just in case object is nil).
can :read, :all do |object_class, object| object_class != Order end
Here the user has permission to read all objects except orders.
You can also pass :manage as the action which will match any action. In this case the action is passed to the block.
can :manage, Comment do |action, comment| action != :destroy end
Finally, you can use the “alias_action” method to alias one or more actions into one.
alias_action :update, :destroy, :to => :modify can :modify, Comment
The following aliases are added by default for conveniently mapping common controller actions.
alias_action :index, :show, :to => :read alias_action :new, :to => :create alias_action :edit, :to => :update
Use the “can?” method in the controller or view to check the user's permission for a given action and object.
can? :destroy, @project
You can also pass the class instead of an instance (if you don't have one handy).
<% if can? :create, Project %> <%= link_to "New Project", new_project_path %> <% end %>
The “cannot?” method is for convenience and performs the opposite check of “can?”
cannot? :destroy, @project
You can have fine grained control over abilities by coming up with new actions. For example, if only pro users are allowed to upload a picture for their product, you could add the following restrictions.
# ability.rb can :upload_picture, Project if user.pro? # projects/_form.html.erb <%= f.file_field :picture if can? :upload_picture, @project %> # projects_controller.rb def update unauthorized! if params[:project][:upload_picture] && cannot?(:upload_picture, @project) # ... end
Assumptions & Configuring
CanCan makes two assumptions about your application.
You have an Ability class which defines the permissions.
You have a current_user method in the controller which returns the current user model.
You can override these by overriding the “current_ability” method in your ApplicationController.
def current_ability UserAbility.new(current_account) # instead of Ability.new(current_user) end
Permissions in Database
Perhaps a non-coder needs the ability to modify the user abilities, or you want to change them without having to re-deploy the application. In that case it may be best to store the permission logic in a separate model, let's call it Permission. It is easy to use the database records when defining abilities.
For example, let's assume that each user has_many :permissions, and each permission has “action”, “object_type” and “object_id” columns. The last of which is optional.
class Ability include CanCan::Ability def initialize(user) can :manage, :all do |action, object_class, object| user.permissions.find_all_by_action(action).any? do |permission| permission.object_type == object_class.to_s && (object.nil? || permission.object_id.nil? || permission.object_id == object.id) end end end end
An alternatie approach is to define a separate “can” ability for each permission.
user.permissions.each do |permission| can permission.action, permission.object_type.constantize do |object| object.nil? || permission.object_id.nil? || permission.object_id == object.id end end end
The actual details will depend largely on your application requirements, but hopefully you can see how it's possible to define permissions in the database and use them with CanCan.
It is very easy to test the Ability model since you can call “can?” directly on it as you would in the view or controller.
def test “user can only destroy projects which he owns”
user = User.new ability = Ability.new(user) assert ability.can?(:destroy, Project.new(:user => user)) assert ability.cannot?(:destroy, Project.new)