= CanCan

Wiki[http://wiki.github.com/ryanb/cancan] | RDocs[http://rdoc.info/projects/ryanb/cancan] | Screencast[http://railscasts.com/episodes/192-authorization-with-cancan]

CanCan is an authorization solution for Ruby on Rails for restricting what a given user is allowed to access throughout the application. It does not care how your user roles are defined, it simply focusses on keeping permission logic in a single location (the +Ability+ class) so it is not duplicated across controllers, views, and database queries.

By default, the +current_user+ method is required, so 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.


== Installation

To install CanCan, include the gem in the environment.rb in Rails 2.3.

  config.gem "cancan"

Or the Gemfile in Rails 3.

  gem "cancan"

Alternatively it can be installed as a plugin.

  script/plugin install git://github.com/ryanb/cancan.git


== Getting Started

First, define a class called +Ability+ in "models/ability.rb" or anywhere else in the load path. 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}[http://wiki.github.com/ryanb/cancan/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}[http://wiki.github.com/ryanb/cancan/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}[http://wiki.github.com/ryanb/cancan/exception-handling] for more information.


== 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 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}[http://wiki.github.com/ryanb/cancan/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}[http://wiki.github.com/ryanb/cancan/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}[http://wiki.github.com/ryanb/cancan/defining-abilities-with-blocks] for more information.


== Aliasing Actions

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

The +alias_action+ method is an instance method and usually called in +initialize+. See {Custom Actions}[http://wiki.github.com/ryanb/cancan/custom-actions] for information on adding other actions.


== Fetching Records

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)

This will only work when abilities are defined using hash conditions, not blocks. See {Fetching Records}[http://wiki.github.com/ryanb/cancan/fetching-records] for more information.


== Additional Docs

* {Upgrading to 1.3}[http://wiki.github.com/ryanb/cancan/upgrading-to-13]
* {Nested Resources}[http://wiki.github.com/ryanb/cancan/nested-resources]
* {Testing Abilities}[http://wiki.github.com/ryanb/cancan/testing-abilities]
* {Accessing Request Data}[http://wiki.github.com/ryanb/cancan/accessing-request-data]
* {Admin Namespace}[http://wiki.github.com/ryanb/cancan/admin-namespace]
* {See more}[http://wiki.github.com/ryanb/cancan/]

== Special Thanks

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.