Redirect to action after authentication

application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery with: :exception

private

  def signed_in?
    !!session[:user_id]
  end
  helper_method :signed_in?
  
  def current_user
    return unless signed_in?
    @_current_user ||= User.find(session[:user_id])
  end
  helper_method :current_user

  def current_user=(user)
    session[:user_id] = user.id
  end

  def authenticate
    return_to signin_url(return_to) unless signed_in?
  end

  def return_to
    { return_to: request.url } if request.get?
  end
  helper_method :return_to
end

index.html.erb

<% unless signed_in? %>
  <%= link_to "Sign In", signin_path(return_to) %>
<% end %>

items_controller.rb

class ItemsController < ApplicationController
  before_action :authenticate

  def new
    @item = Item.new
  end
end

new.html.erb

<%= form_tag signin_path(return_to: params[:return_to]) do %>
  <%= label_tag :email %>
  <%= email_field_tag :email %>

  <%= label_tag :password %>
  <%= password_field_tag :password %>

  <%= submit_tag "Log In" %>
<% end %>

sessions_controller.rb

class SessionsController < ApplicationController
  def create
    user = User.find_by(email: params[:email])

    if user && user.authenticate(params[:password])
      self.current_user = user
      redirect_to onsite_url(params[:return_to]) || root_url
    else
      flash.now.alert = 'Wrong email/password combination'
      render :new
    end
  end
  
  private
  
  def onsite_url(url)
    URI.parse(url).path if url
    # URI.join(root_url, URI.parse(url).path).to_s if url
  end
end