current_user helpers without Devise

application_controller.rb

class ApplicationController < ActionController::Base
  include SessionsHelper

  Unauthorized = Class.new(StandardError)

  def authorize_user!
    raise Unauthorized unless current_user
  end
end

routes.rb

# ...

match '/auth/logout',             to: 'sessions#destroy',      via: [:get, :delete], as: 'logout'
match '/auth/failure',            to: 'sessions#auth_failure', via: [:get, :post]
match '/error',                   to: 'sessions#auth_failure', via: [:get, :post]
match '/auth/:provider/callback', to: 'sessions#create',       via: [:get, :post], as: :auth_callback

# Omniauth login with provider path: auth_path('twitter')
get   '/auth/:provider', to: -> env { [404, {}, ["Not Found"]] }, as: 'auth'

# ...

sessions_helper.rb

module SessionsHelper

  def sign_in(user)
    reset_session
    self.current_user = user
  end

  def sign_out
    self.current_user = nil
    reset_session
  end

  def signed_in?
    !current_user.nil?
  end

  def current_user=(user)
    @current_user = user
    if user.present? && user.persisted?
      session[:user_id] = user.id
    else
      session.delete(:user_id)
      nil
    end
  end

  def current_user
    @current_user ||= User.find_by(id: session[:user_id])

  rescue ActiveRecord::RecordNotFound
    self.current_user = nil
  end

end