gistfile1.rb

module Authentication
  def logged_in?
    current_user.is_a?(User)
  end

  def current_user
    @current_user ||= User.find_by_id(session[:user_id]) || :false
  end

  def current_user=(new_user)
    if (new_user && new_user.is_a?(User)
      session[:user_id] = new_user.id
      @current_user = new_user
    else
      session[:user_id] = nil
      @current_user = :false
    end
  end

  # Inclusion hook to make #current_user and #logged_in?
  # available as ActionView helper methods.
  def self.included(base)
    base.send :helper_method, :current_user, :logged_in?
  end
end

class ApplicationController < ActionController::Base
  include Authentication
end

class SessionController < ApplicationController
  def destroy
    reset_session # teoricamente, nem precisaria, pois no current_user= eu atribuo nil para session[:user_id]
    self.current_user = nil
    redirect_to root_url
  end
end

# Já tentei session[:user_id] = nil