Add test for stock changes

test/integration/stock_test.rb

@@ -0,0 +1,16 @@
+require 'test_helper'
+
+class StockTest < ActiveSupport::TestCase
+  test "creating and deleting orders updates stock of products" do
+    order = users(:benji).orders.build
+    order.order_items.build(product: products(:fanta), count: 2)
+
+    assert_difference "products(:fanta).stock", -2 do
+      order.save(validate: false)
+    end
+
+    assert_difference "products(:fanta).stock", +2 do
+      order.destroy
+    end
+  end
+end