Exercise 1 solution: DiscountedTotalInCents on the domain order + integration tests

Domain-only field: OrderDto and OrderData stay without it; the stored row
keeps the original total after saving a discounted order. The discount
percentage is not copied (derivable).

yak: exercise-1-solution-on-branch-exercise-1-solution-4s5q

src/After/Order.cs

@@ -24,6 +24,15 @@ public class Order
     /// </summary>
     public int TotalInCents { get; set; }
 
+    /// <summary>
+    /// The discounted total in cents, copied onto the order by the caller of
+    /// the discount flow (see <see cref="DiscountCalculator"/>). Derived
+    /// data: it lives in the domain only — neither <see cref="OrderDto"/>
+    /// nor <see cref="OrderData"/> carries it — and the applied percentage
+    /// is not stored either, being derivable from the two totals.
+    /// </summary>
+    public int DiscountedTotalInCents { get; set; }
+
     /// <summary>
     /// The client that owns this order. Set automatically by
     /// <see cref="ClientOrders"/> when the order is added to

tests/BeforeAfter.Tests/DiscountedOrderMappingTests.cs

@@ -0,0 +1,59 @@
+using After;
+using static BeforeAfter.Tests.TestData.ClientBuilder;
+using static BeforeAfter.Tests.TestData.OrderBuilder;
+
+namespace BeforeAfter.Tests;
+
+/// <summary>
+/// EXERCISE — the mapping from a <see cref="DiscountedOrder"/> onto an
+/// <see cref="Order"/> is NOT one-on-one. (This is the solution participants
+/// arrive at from the stub in EXERCISE.md.)
+/// </summary>
+public class DiscountedOrderMappingTests
+{
+    [Fact] // EXERCISE STEP 2
+    public void A_discounted_order_can_be_correlated_onto_its_order()
+    {
+        var db = new DbContext();
+        var order = ValidOrder().WithDescription("big order").WithTotal(150_00).Build();
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
+        db.Save(client);
+        db.Save(order);
+
+        // 10 % for orders above €100:
+        var discounted = DiscountCalculator.CalculateDiscount(
+            order, discountBarrierCents: 100_00, discountPercentage: 10);
+
+        // Correlate: the discounted result belongs to the order carrying this id.
+        Assert.Equal(order.Id, discounted.OrderId);
+
+        // Copy the discounted total onto the order. The discount percentage is
+        // deliberately NOT copied — it is derivable from the two totals.
+        order.DiscountedTotalInCents = discounted.DiscountedTotalInCents;
+
+        Assert.Equal(135_00, order.DiscountedTotalInCents);
+    }
+
+    [Fact] // EXERCISE STEP 3
+    public void Saving_a_discounted_order_keeps_the_stored_total_untouched()
+    {
+        var db = new DbContext();
+        var order = ValidOrder().WithDescription("big order").WithTotal(150_00).Build();
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
+        db.Save(client);
+        db.Save(order);
+
+        var discounted = DiscountCalculator.CalculateDiscount(
+            order, discountBarrierCents: 100_00, discountPercentage: 10);
+        Assert.Equal(order.Id, discounted.OrderId);
+        order.DiscountedTotalInCents = discounted.DiscountedTotalInCents;
+
+        // Save the updated order: the context refreshes the stored row in place.
+        db.Save(order);
+
+        // The persisted record keeps the ORIGINAL total: the database knows
+        // nothing about discounts — derived data lives in the domain only.
+        var row = db.Find<OrderData>(candidate => candidate.Id == discounted.OrderId)!;
+        Assert.Equal(150_00, row.TotalInCents);
+    }
+}