I’m getting the same thing. “rails -v” shows “Rails 4.0.0.beta1”. “dir app\controllers\concerns” shows that the folder exists with one file, “current_cart.rb”.
Contents of current_cart.rb and line_items_controller.rb match exactly to the source on media.pragprog.com.
The first time I hit “Add to Cart” I get the uninitialized constant error he mentions above. But if I hit back and then “Add to Cart” again, the second (and third, fourth…) time I get:
The action ‘create’ could not be found for LineItemsController.
I’ve tried all sorts of other things too but can’t get it to work. Originally I had this app set up under Rails 3.2.13 and was getting this error, so I upgraded to 4.0.0.beta1 to try and fix it (and overwrote all necessary files) but still can’t get it working.