Iteration E2.

app/controllers/carts_controller.rb

@@ -1,5 +1,7 @@
 class CartsController < ApplicationController
   before_action :set_cart, only: [:show, :edit, :update, :destroy]
+  # QUESTION: How do we call this with call?
+  rescue_from ActiveRecord::RecordNotFound, with: :invalid_cart
 
   # GET /carts
   # GET /carts.json
@@ -71,4 +73,9 @@ class CartsController < ApplicationController
     def cart_params
       params.fetch(:cart, {})
     end
+
+    def invalid_cart
+      logger.error "Attempt to access invalid cart #{params[:id]}"
+      redirect_to store_index_url, notice: 'Invalid cart'
+    end
 end

app/controllers/line_items_controller.rb

@@ -74,6 +74,6 @@ class LineItemsController < ApplicationController
 
     # Never trust parameters from the scary internet, only allow the white list through.
     def line_item_params
-      params.require(:line_item).permit(:product_id, :cart_id)
+      params.require(:line_item).permit(:product_id)
     end
 end

test/controllers/line_items_controller_test.rb

@@ -37,7 +37,8 @@ class LineItemsControllerTest < ActionDispatch::IntegrationTest
   end
 
   test "should update line_item" do
-    patch line_item_url(@line_item), params: { line_item: { cart_id: @line_item.cart_id, product_id: @line_item.product_id } }
+    patch line_item_url(@line_item),
+          params: { line_item: { product_id: @line_item.product_id } }
     assert_redirected_to line_item_url(@line_item)
   end