Example of a Rails api authentication module using ActiveSupport::Concern

authenticator.rb

module Api::V1::Concerns
  module Authenticator
    extend ActiveSupport::Concern

    included do
      before_action :auth_with_token!
    end

    def current_user
      @current_user ||= User.find_by(auth_token: request.headers['Authorization'])
    end

    def user_signed_in?
      current_user.present?
    end

    def auth_with_token!
      head :unauthorized unless user_signed_in?
    end
  end
end