= CanCan

This is a simple authorization solution for Rails which is completely decoupled from how you set up the user's roles. All permissions are stored in a single location for convenience.

This assumes you already have an authentication solution (such as Authlogic) which proves 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, place it in "models/ability.rb".

  class Ability
    include CanCan::Ability
  
    def prepare(user)
      if user.admin?
        can :manage, :all
      else
        can :read, :all
      end
    end
  end

This class is where all permissions will go. See the "Defining Abilities" section below for more information.

In the view layer you can access the current permissions at any point using the "can?" method. See "Checking Abilities" section below.

  <% if can? :update, @article %>
    <%= link_to "Edit", edit_article_path(@article) %>
  <% end %>

You can also use this method in the controller layer along with the "unauthorized!" method to restrict access.

  def show
    @article = Article.find(params[:id])
    unauthorized! unless can? :read, @article
  end

Setting this for every action can be tedious, therefore a before filter is also provided for automatically applying this setting to 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.

  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#prepare method is where all user permissions are defined. The user model is passed into this 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. For example:

  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). For example:

  <% if can? :create, Project %>
    <%= link_to "New Project", new_project_path %>
  <% end %>


== Custom Actions

There is no limit to what actions you can use to determine abilities. For example, if only pro users are allowed to upload a picture for their product, you might add restrictions like this.

  # 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] && !can?(:upload_picture, @project)
    # ...
  end


== Customizing Assumptions

CanCan makes two assumptions about your application.

* The permissions are defined in Ability#prepare.
* The user is fetched with current_user method in the controller.

You can override these by defining the "current_ability" method in your ApplicationController.

   def current_ability
     ability = UserAbility.new          # instead of Ability
     ability.prepare(current_account)   # instead of current_user
     ability                            # be sure to return the ability
   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 prepare(user)
      can :manage, :all do |action, object_class, object|
        user.permissions.find_all_by_action(action).any? do |permission|
          permission.object_type.constantize == object_class &&
            (object.nil? || permission.object_id.nil? || permission.object_id == object.id)
        end
      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.


== Special Thanks

CanCan was inspired by declarative_authorization[http://github.com/stffn/declarative_authorization/] and aegis[http://github.com/makandra/aegis]. Many thanks to the authors and contributors.