Allow setting discount for an empty order

rails_application/app/controllers/orders_controller.rb

@@ -36,7 +36,8 @@ def edit_discount
 
   def update_discount
     @order_id = params[:id]
-    if Orders::Order.find_by_uid(params[:id]).percentage_discount
+    order = Orders::Order.find_or_create_by!(uid: params[:id])
+    if order.percentage_discount
       command_bus.(Pricing::ChangePercentageDiscount.new(order_id: @order_id, amount: params[:amount]))
     else
       command_bus.(Pricing::SetPercentageDiscount.new(order_id: @order_id, amount: params[:amount]))

rails_application/app/read_models/client_orders/update_discount.rb

@@ -1,7 +1,7 @@
 module ClientOrders
   class UpdateDiscount < Infra::EventHandler
     def call(event)
-      order = Order.find_by(order_uid: event.data.fetch(:order_id))
+      order = Order.find_or_create_by!(order_uid: event.data.fetch(:order_id))
       order.percentage_discount = event.data.fetch(:amount)
       order.save!
     end

rails_application/test/integration/orders_test.rb

@@ -183,6 +183,31 @@ def test_order_value_doesnt_change_after_changing_price
     assert_select("td", text: "$78.00")
   end
 
+  def test_discount_is_applied_for_new_order
+    order_id = SecureRandom.uuid
+    async_remote_id = register_product("Async Remote", 39, 10)
+    fearless_id     = register_product("Fearless Refactoring", 49, 10)
+    shopify_id = register_customer("Shopify")
+
+    assert_nothing_raised { apply_discount_10_percent(order_id)}
+
+    post "/orders/#{order_id}/add_item?product_id=#{async_remote_id}"
+    post "/orders/#{order_id}/add_item?product_id=#{fearless_id}"
+
+    post "/orders",
+         params: {
+           "authenticity_token" => "[FILTERED]",
+           "order_id" => order_id,
+           "customer_id" => shopify_id,
+           "commit" => "Submit order"
+         }
+
+    follow_redirect!
+    assert_select("td", "$79.20")
+    assert_select("dd", "Submitted")
+    assert_select("td", "10.0%")
+  end
+
   private
 
   def verify_shipping(order_id)