How to use devise and cancancan gem instraction:

Get video tutorial:

First Step

create a new rails app

terminal

$ rails new devise_cancan

$ rails g scaffold status post:text

add gem to gemfile

\Gemfile

gem 'devise'
gem 'cancancan'

Second Step

setup devise

terminal

$ rails generate devise:install

\config\environments\development.rb

 config.action_mailer.default_url_options = { host: 'localhost', port: 3000 }

\config\routes.rb

 root 'statuses#index'

terminal

$ rails generate devise User

$ rake db:migrate

$ rails generate devise:views

\app\controllers\statuses_controller.rb

 before_action :authenticate_user!

\app\views\layouts\application.html.erb

<% if user_signed_in? %>
    <%= link_to "sign out",destroy_user_session_path, method: :delete   %>
<% end %>
<br>

Third step

setup cancancan abilities:

terminal

$ rails g cancan:ability

$ rails generate migration add_roles_mask_to_users roles_mask:integer

$ rake db:migrate

\app\models\user.rb

ROLES = %i[admin author]

def roles=(roles)
  roles = [*roles].map { |r| r.to_sym }
  self.roles_mask = (roles & ROLES).map { |r| 2**ROLES.index(r) }.inject(0, :+)
end

def roles
  ROLES.reject do |r|
    ((roles_mask.to_i || 0) & 2**ROLES.index(r)).zero?
  end
end

def has_role?(role)
  roles.include?(role)
end

\app\controller\application_controller.rb

before_action :configure_permitted_parameters, if: :devise_controller?
protected

def configure_permitted_parameters
  devise_parameter_sanitizer.for(:sign_up)  { |u| u.permit(  :email,:password, :password_confirmation, roles: []) }
end

\app\views\devise\registrations\new.html.erb

<% for role in User::ROLES %>
 <%= check_box_tag "user[roles][#{role}]", role, @user.roles.include?(role), {:name => "user[roles][]"}%>
 <%= label_tag "user_roles_#{role}", role.to_s.humanize %><br />
<% end %>
<%= hidden_field_tag "user[roles][]", "" %>

\app\models\ability.rb

  user ||= User.new
  if user.has_role? :admin
    can :manage, :all
  elsif user.has_role? :author
    can :create, Status # author can create status
    can :update, Status # author can update status
    # can :destroy, Status # #uncomment this line, author can destroy status 
    can :read, :all
  else
    can :read, :all
  end

\app\controllers\statuses_controller.rb

 load_and_authorize_resource

Now Test It !!