current_user.rb

module CurrentUser
  def signed_in?
    !current_user.nil?
  end

  def sign_in(user)
    self.current_user, session["user_id"] = user, user.id
  end

  def sign_out
    self.current_user = session["user_id"] = nil
  end

  def current_user=(user)
    @current_user = user
  end

  def current_user
    @current_user || User.find(session["user_id"]) if session["user_id"] || nil
  end
end