In almost every web application there are certain things which a user is allowed to do only after he or she is logged  in into the website. If user  trying to access some page or some URL where he/she can get access only after logging into  the website then he should be redirected back to the same page as soon as he logs in after filling in his ID and Password. 

To achieve this functionality we can use the requested path and save it to be used again once the user is logged in.

If we are using the gem Devise our authentication purposes then a way to achieve this functionality is as follows:

For example, if we are creating an e-commerce application where a user can add some products to his cart for shopping purposes, then he or she is redirected to login page to add the item to his cart. 

For this, as soon as the user will press add to cart button, the control will go the cart controller where we will call

these two methods on the top :

class CartsController < ApplicationController
  before_action :store_location,:authenticate_user!
  
  def create

  end

  ....

end

This shows us that as soon as a request will come to the carts controller the before action will call these two methods. The authenticate user method will take the user to the login form to log into the web site and store location method will be defined in the application controller in which we will save our previous URL

class ApplicationController < ActionController::Base 

 def store_location
    if(request.path != "/users/sign_in" &&
      request.path != "/users/sign_up" &&
      request.path != "/users/password/new" &&
      request.path != "/users/password/edit" &&
      request.path != "/users/confirmation" &&
      request.path != "/users/sign_out" &&
      !request.xhr? && !current_user) # don't store ajax calls
      session[:previous_url] = request.fullpath
    end
  end

  def after_sign_in_path_for(resource)
    previous_path = session[:previous_url]
    session[:previous_url] = nil
    previous_path || root_path
  end

  ...

end

The store location method will save the full URL in a session hash. This hash is used in the after sign in path method of a device which is used to redirect a user to a particular URL after sign in. In this method if the session hash receives a value of a previous URL then it is redirected to that page or it will be redirected to the root path.

So this is how we can redirect a user back to the same page from where he was redirected to login page.