A Cut Above: CanCanCan

February 9, 2017
gem rails ruby web

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Developing a web application with multiple user types can lead to tons of headaches about authorization. Trying to come up with my own solution led to lots of logic in before_actions in controllers, and even more awkward logic in views shared by multiple user types that required slight differences. This mess also extended throughout multiple files, making it even worse.

Fed up, I found a gem called CanCanCan that made authorization so much simpler. Rather than lots of checks spread throughout your codebase, CanCanCan keeps it all together with an intuitive DSL.

I’ve set up a typical example below. I’d normally explain a code segment before, but I think CanCanCan’s DSL is so easy to read that it does not require anything but the few comments.

# app/models/ability.rb

class Ability
  include CanCan::Ability
  
  def initialize(user)
    alias_action :create, :read, :update, :destroy, to: :crud
    
    if user.is_admin? or user.is_moderator?
      # Admins and mods can CRUD any post
      can :crud, Post
    else
      # Normal users can CRUD their own post
      can :crud, Post, user_id: user.id
      # But they can only read others
      can :read, Post
    end
  end
end
# app/controllers/posts_controller

class PostsController < ApplicationController
  load_and_authorize_resource
end

And that’s it as far as controllers go. What about views, though?

Let’s say you want admins and moderators to be the only ones to see the delete button for the post (within the post partial).

# app/views/posts/_post.html.erb

<% if can? :destroy, post %>
  <%= link_to 'Delete', post, method: :delete, class: 'btn btn-danger' %>
<% end %>

This is so much better than checking a user’s class without understanding what the context for the check is.

Something to remember, though: CanCanCan’s automagic-ness relies on standard Rails naming practices. If, however, you find yourself breaking the standard a bit, the gem offers plenty of options to work with almost any situation.

I can not recommend this gem enough to anybody designing a web app with multiple user types. CanCanCan is a real gem of a gem, especially if you use Devise.