CanCan

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.

Installation

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

Setup

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

Defining Abilities

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

Checking Abilities

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

Custom Actions

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

That’s it!

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.

def initialize(user)

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.

Testing Abilities

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)

end

Special Thanks

CanCan was inspired by declarative_authorization and aegis. Many thanks to the authors and contributors.