access granted

Multi-role and whitelist based authorization gem for Rails (and not only Rails!)

chaps-io
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Ruby

AccessGranted Code Climate

AccessGranted is a multi-role and whitelist based authorization gem for Rails. And it’s lightweight (~300 lines of code)!

Installation

Add the gem to your gemfile:

gem 'access-granted', '~> 1.3'

Run the bundle command to install it. Then run the generator:

rails generate access_granted:policy

Supported Ruby versions

Because it has zero runtime dependencies it is guaranteed to work on all supported MRI Ruby versions, see CI to check the up to date list.
It might and probably is working on Rubinius and JRuby but we are no longer testing against those.

Summary

AccessGranted is meant as a replacement for CanCan to solve major problems:

  1. Performance

    On average AccessGranted is 20 times faster in resolving identical permissions and takes less memory.
    See benchmarks.

  2. Roles

    Adds support for roles, so no more ifs and elses in your Policy file. This makes it extremely easy to maintain and read the code.

  3. Whitelists

    This means that you define what the user can do, which results in clean, readable policies regardless of application complexity.
    You don’t have to worry about juggling cans and cannots in a very convoluted way!

    Note: cannot is still available, but has a very specifc use. See Usage below.

  4. Framework agnostic

    Permissions can work on basically any object and AccessGranted is framework-agnostic,
    but it has Rails support out of the box. 😃
    It does not depend on any libraries, pure and clean Ruby code. Guaranteed to always work,
    even when software around changes.

Usage

Roles are defined using blocks (or by passing custom classes to keep things tidy).

Order of the roles is VERY important, because they are being traversed in top-to-bottom order.
At the top you must have an admin or some other important role giving the user top permissions, and as you go down you define less-privileged roles.

I recommend starting your adventure by reading the wiki page on how to start with Access Granted, where I demonstrate its abilities on a real life example.

Defining an access policy

Let’s start with a complete example of what can be achieved:

# app/policies/access_policy.rb

class AccessPolicy
  include AccessGranted::Policy

  def configure
    # The most important admin role, gets checked first
    role :admin, { is_admin: true } do
      can :manage, Post
      can :manage, Comment
    end

    # Less privileged moderator role
    role :moderator, proc {|u| u.moderator? } do
      can [:update, :destroy], Post
      can :update, User
    end

    # The basic role. Applies to every user.
    role :member do
      can :create, Post

      can [:update, :destroy], Post do |post, user|
        post.author == user && post.comments.empty?
      end
    end
  end
end

Defining roles

Each role method accepts the name of the role you’re creating and an optional matcher.
Matchers are used to check if the user belongs to that role and if the permissions inside should be executed against it.

The simplest role can be defined as follows:

role :member do
  can :read, Post
  can :create, Post
end

This role will allow everyone (since we didn’t supply a matcher) to read and create posts.

But now we want to let admins delete those posts.
In this case we can create a new role above the :member to add more permissions for the admin:

role :admin, { is_admin: true } do
  can :destroy, Post
end

role :member do
  can :read, Post
  can :create, Post
end

The { is_admin: true } hash is compared with the user’s attributes to see if the role should be applied to it.
So, if the user has an attribute is_admin set to true, then the role will be applied to it.

Note: you can use more keys in the hash to check many attributes at once.

Hash conditions

Hashes can be used as matchers to check if an action is permitted.
For example, we may allow users to only see published posts, like this:

role :member do
  can :read, Post, { published: true }
end

Block conditions

Sometimes you may need to dynamically check for ownership or other conditions,
this can be done using a block condition in can method, like so:

role :member do
  can :update, Post do |post, user|
    post.author_id == user.id
  end
end

When the given block evaluates to true, then user is allowed to update the post.

Roles in order of importance

Additionally, we can allow admins to update all posts despite them not being authors like so:

role :admin, { is_admin: true } do
  can :update, Post
end

role :member do
  can :update, Post do |post, user|
    post.author_id == user.id
  end
end

As stated before: :admin role takes precedence over :member role, so when AccessGranted sees that admin can update all posts, it stops looking at the less important roles.

That way you can keep a tidy and readable policy file which is basically human readable.

Usage with Rails

AccessGranted comes with a set of helpers available in Ruby on Rails apps:

Authorizing controller actions

class PostsController
  def show
    @post = Post.find(params[:id])
    authorize! :read, @post
  end

  def create
    authorize! :create, Post
    # (...)
  end
end

authorize! throws an exception when current_user doesn’t have a given permission.
You can rescue from it using rescue_from:

class ApplicationController < ActionController::Base
  rescue_from "AccessGranted::AccessDenied" do |exception|
    redirect_to root_path, alert: "You don't have permission to access this page."
  end
end

You can also extract the action and subject which raised the error,
if you want to handle authorization errors differently for some cases:

  rescue_from "AccessGranted::AccessDenied" do |exception|
    status = case exception.action
      when :read # invocation like `authorize! :read, @something`
        403
      else
        404
      end

    body = case exception.subject
      when Post # invocation like `authorize! @some_action, Post`
        "failed to access a post"
      else
        "failed to access something else"
      end
  end

You can also have a custom exception message while authorizing a request.
This message will be associated with the exception object thrown.

class PostsController
  def show
    @post = Post.find(params[:id])
    authorize! :read, @post, 'You do not have access to this post'
    render json: { post: @post }
  rescue AccessGranted::AccessDenied => e
    render json: { error: e.message }, status: :forbidden
  end
end

Checking permissions in controllers

To check if the user has a permission to perform an action, use the can? and cannot? methods.

Example:

class UsersController
  def update
    # (...)

    # only admins can elevate users to moderator status

    if can? :make_moderator, @user
      @user.moderator = params[:user][:moderator]
    end

    # (...)
  end
end

Checking permissions in views

Usually you don’t want to show “Create” buttons for people who can’t create something.
You can hide any part of the page from users without permissions like this:

# app/views/categories/index.html.erb

<% if can? :create, Category %>
  <%= link_to "Create new category", new_category_path %>
<% end %>

Customizing policy

By default, AccessGranted adds this method to your controllers:

  def current_policy
    @current_policy ||= ::AccessPolicy.new(current_user)
  end

If you have a different policy class or if your user is not stored in the current_user variable, then you can override it in any controller and modify the logic as you please.

You can even have different policies for different controllers!

Usage with pure Ruby

Initialize the Policy class:

policy = AccessPolicy.new(current_user)

Check the ability to do something:

with can?:

policy.can?(:create, Post) #=> true
policy.can?(:update, @post) #=> false

or with cannot?:

policy.cannot?(:create, Post) #=> false
policy.cannot?(:update, @post) #=> true

Common examples

Extracting roles to separate files

Let’s say your app is getting bigger and more complex. This means your policy file is also getting longer.

Below you can see an extracted :member role:

class AccessPolicy
  include AccessGranted::Policy

  def configure
    role :administrator, is_admin: true do
      can :manage, User
    end

    role :member, MemberRole, -> { |user| !u.guest? }
  end
end

And roles should look like this:

# app/roles/member_role.rb

class MemberRole < AccessGranted::Role
  def configure
    can :create, Post
    can :destroy, Post do |post, user|
      post.author == user
    end
  end
end

Compatibility with CanCan

This gem has been created as a replacement for CanCan and therefore it requires minimum work to switch.

Main differences

  1. AccessGranted does not extend ActiveRecord in any way, so it does not have the accessible_by?
    method which could be used for querying objects available to current user.
    This was very complex and only worked with permissions defined using hash conditions, so
    I decided to not implement this functionality as it was mostly ignored by CanCan users.

  2. Both can?/cannot? and authorize! methods work in Rails controllers and views, just like in CanCan.
    The only change you have to make is to replace all can? :manage, Class with the exact action to check against.
    can :manage is still available for defining permissions and serves as a shortcut for defining :create, :read, :update, :destroy all in one line.

  3. Syntax for defining permissions in the AccessPolicy file (Ability in CanCan) is exactly the same,
    with roles added on top. See Usage above.

Contributing

  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new pull request