Ruby Grape API authorization with Pundit example, without any extra gems

my_grape_api.rb

class MyGrapeAPI < Grape::API
  helpers Pundit
  after { verify_authorized }

  helpers do
    def current_user
      nil # your authentication mechanism
    end
  end

  resources :users do
    desc "Retrieves information about given user"
    params do
      requires :id, type: String, desc: 'User ID'
    end
    get ':id' do
      user = User.find(params[:id])
      authorize user, :show?
      present user, with: Entities::User
    end
  end
end

I created an authorization helper:

# api/app_helpers/authorization_kit.rb
module AppHelpers
  module AuthorizationKit
    def self.included(mod)
      mod.after { verify_authorized }
      mod.helpers Pundit
    end
  end
end

Then I just include it:

class UserAPI < Grape::API
  include AppHelpers::AuthorizationKit

  resources :users do
    # ..
  end
end