If you need a method that will be accessed from both view and controller write the method in controller and pass the name of the function as an argument of helper_method.
class ApplicationController < ActionController::Base
user_id = session[:user_id]
user = User.find_by_id(user_id)
Here, logged_in_user method is accessible from all of the controllers those extend ApplicationController and helper_method :logged_in_user will expose this method to views. Your common accessible method signature may have arguments. But, only helper_method :logged_in_user is enough to make it accessible from view with the same signature defined.