Canable

Simple permissions that I have used on my last several projects so I figured it was time to abstract and wrap up into something more easily reusable.

Cans

Whatever class you want all permissions to run through should include Canable::Cans.

class User
  include MongoMapper::Document
  include Canable::Cans
end

This means that an instance of a user automatically gets can methods for the default REST actions: can_view?(resource), can_create?(resource), can_update?(resource), can_destroy?(resource).

Ables

Each of the can methods simply calls the related “able” method (viewable, creatable, updatable, destroyable) for the action (view, create, update, delete). Canable comes with defaults for this methods that you can then override as makes sense for your permissions.

class Article
  include MongoMapper::Document
  include Canable::Ables
end

Including Canable::Ables adds the able methods to the class including it. In this instance, article now has viewable_by?(user), creatable_by?(user), updatable_by?(user) and destroyable_by?(user).

Lets say an article can be viewed and created by anyone, but only updated or destroyed by the user that created the article. To do that, you could leave viewable_by? and creatable_by? alone as they default to true and just override the other methods.

class Article
  include MongoMapper::Document
  include Canable::Ables
  userstamps! # adds creator and updater

  def updatable_by?(user)
    creator == user
  end

  def destroyable_by?(user)
    updatable_by?(user)
  end
end

Lets look at some sample code now:

john = User.create(:name => 'John')
steve = User.create(:name => 'Steve')

ruby = Article.new(:title => 'Ruby')
john.can_create?(ruby) # true
steve.can_create?(ruby) # true

ruby.creator = john
ruby.save

john.can_view?(ruby) # true
steve.can_view?(ruby) # true

john.can_update?(ruby) # true
steve.can_update?(ruby) # false

john.can_destroy?(ruby) # true
steve.can_destroy?(ruby) # false

Now we can implement our permissions for each resource and then always check whether a user can or cannot do something. This makes it all really easy to test. Next, how would you use this in the controller.

Enforcers

class ApplicationController
  include Canable::Enforcers
end

Including Canable::Enforcers adds an enforce permission method for each of the actions defined (by default view/create/update/destroy). It is the same thing as doing this for each Canable action:

class ApplicationController
  include Canable::Enforcers

  delegate :can_view?, :to => :current_user
  helper_method :can_view? # so you can use it in your views
  hide_action :can_view?

  private
    def enforce_view_permission(resource)
      raise Canable::Transgression unless can_view?(resource)
    end
end

Which means you can use it like this:

class ArticlesController < ApplicationController
  def show
    @article = Article.find!(params[:id])
    enforce_view_permission(@article)
  end
end

If the user can_view? the article, all is well. If not, a Canable::Transgression is raised which you can decide how to handle (show 404, slap them on the wrist, etc.).

Adding Your Own Actions

You can add your own actions like this:

Canable.add(:publish, :publishable)

The first parameter is the can method (ie: can_publish?) and the second is the able method (ie: publishable_by?).

Ables can also be added as class methods. For example, to restrict access to an index action:

Canable.add(:index, :indexable)

Then enforce by passing the class instead of the instance:

class ArticlesController < ApplicationController
  def index
    @articles = Article.all
    enforce_index_permission(Article)
  end
end

Then in the article model, add the able check as a class method:

class Article
  ...
  def self.indexable_by?(user)
    !user.nil?
  end
end

Review

So, lets review: cans go on user model, ables go on everything, you override ables in each model where you want to enforce permissions, and enforcers go after each time you find or initialize an object in a controller. Bing, bang, boom.

Note on Patches/Pull Requests

Fork the project.
Make your feature addition or bug fix.
Add tests for it. This is important so I don’t break it in a future version unintentionally.
Commit, do not mess with rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
Send me a pull request. Bonus points for topic branches.