point facilitator notes at the two solution branches

Steps: (1) OrderLine + computed total — the compiler enumerates the writers,
(2) one-line reconstruction in the two inbound mappers, (3) discount on a
reloaded order + the accepted-loss test. Dry-run verified on a scratch
branch: 98 tests green, diff confined to domain/builder/two mappers.

yak: exercise2md-orderlines-evolution-sequenced-accepted-loss-test-efap

EXERCISE.md

@@ -22,6 +22,11 @@ explicit — and that is where this exercise lives.
   percentage *actually applied* (0 when the total does not strictly exceed the
   barrier), and the discounted total. Rounding is half-up, in integer math.
 - Unit tests for all of the above already pass.
+- `ValidOrder()` / `ValidClient()` test data builders in the test project
+  (`tests/BeforeAfter.Tests/TestData/`) — construct domain objects in tests
+  through them, so tests survive domain evolution. Import their static
+  factories with `using static BeforeAfter.Tests.TestData.OrderBuilder;`
+  (and `...ClientBuilder;`).
 
 ## The challenge
 
@@ -61,6 +66,8 @@ instead of assuming there is only one order around.
 
 ```csharp
 using After;
+using static BeforeAfter.Tests.TestData.ClientBuilder;
+using static BeforeAfter.Tests.TestData.OrderBuilder;
 
 namespace BeforeAfter.Tests;
 
@@ -74,9 +81,8 @@ public class DiscountedOrderMappingTests
     public void A_discounted_order_can_be_correlated_onto_its_order()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
+        var order = ValidOrder().WithDescription("big order").WithTotal(150_00).Build();
-        var order = new Order { Description = "big order", TotalInCents = 150_00 };
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
-        client.Orders.Add(order);
         db.Save(client);
         db.Save(order);
 
@@ -98,9 +104,8 @@ public class DiscountedOrderMappingTests
     public void Saving_a_discounted_order_keeps_the_stored_total_untouched()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
+        var order = ValidOrder().WithDescription("big order").WithTotal(150_00).Build();
-        var order = new Order { Description = "big order", TotalInCents = 150_00 };
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
-        client.Orders.Add(order);
         db.Save(client);
         db.Save(order);
 

EXERCISE2.md

@@ -0,0 +1,191 @@
+# Exercise 2: OrderLines — the domain evolves, the edges don't
+
+> Run `EXERCISE.md` first (or apply its solution from the
+> `exercise-1-solution` branch): this exercise continues from that state.
+
+You are in the "after" situation: plain domain objects, two mapping families
+out of the domain (DTOs for transport, `*Data` records for persistence), and
+test data builders so tests don't reach into domain construction. The order
+total is a stored `int TotalInCents`.
+
+The scenario: orders should consist of **order lines** — each with its own
+description and amount. The total becomes *derived* data: the sum of the
+lines. And here is the point of the exercise — the persistence framework and
+the DTO keep their flat shape (one `Description`, one `TotalInCents` per
+order), so the domain changes while **nothing at the edges does**: same DTO,
+same storage row, same WebApp.
+
+## The trick, and the accepted loss
+
+Storage holds one flat row per order. Reading an order back therefore
+reconstructs it with **one synthetic order line**: description and amount
+taken from the row's `Description` and `TotalInCents`. Calculation (discount,
+totals) then just works on reloaded orders.
+
+The honest limitation: an order with **more than one line** cannot round-trip
+through storage — saving flattens it, reading back collapses it to one line.
+That is fine as long as multi-line orders never reach the front-end. The
+exercise includes a test that documents this loss instead of hiding it.
+
+Work through the three steps — each is one instruction for a coding agent.
+Run the tests after each step:
+
+```sh
+scripts/run-tests.sh
+```
+
+## Step 1 — introduce OrderLines (and meet the compiler)
+
+In `src/After`:
+
+1. Add an `OrderLine` — a plain, mutable domain type in the same style as
+   `Order`/`Client` (only the DTOs are records):
+
+   ```csharp
+   public class OrderLine(string description, int amountInCents)
+   {
+       public string Description { get; set; } = description;
+       public int AmountInCents { get; set; } = amountInCents;
+   }
+   ```
+
+2. In `Order`: keep `Description` (it stays stored — it is the order-level
+   description), add `public List<OrderLine> Lines { get; } = [];`, and turn
+   the total into a computed property (delete the stored one):
+
+   ```csharp
+   public int TotalInCents => Lines.Sum(line => line.AmountInCents);
+   ```
+
+3. Update `OrderBuilder`: `WithTotal` must keep its API but can no longer set
+   a computed total. Store the request and express it in `Build()` as a single
+   line carrying the order's description — so `WithDescription`/`WithTotal`
+   work in any order:
+
+   ```csharp
+   private int? _totalInCents;
+   public OrderBuilder WithTotal(int totalInCents) { _totalInCents = totalInCents; return this; }
+   public Order Build()
+   {
+       if (_totalInCents is int total)
+           _order.Lines.Add(new OrderLine(_order.Description, total));
+       return _order;
+   }
+   ```
+
+Now build. The compiler enumerates every remaining *writer* of the total —
+exactly two, both **inbound** mappers (`Mappers.FromOrderDto` and
+`Mappers.FromOrderData`). Everything that only *reads* the total —
+`ToOrderDto`, `ToOrderData`, `DiscountCalculator`, the WebApp — compiles
+untouched. That is the whole lesson in one compiler output.
+
+One test also writes the total (`order.TotalInCents = 99_00` in
+`Saving_again_refreshes_the_row_instead_of_duplicating_it`); fix it now by
+mutating through the new domain API instead: `order.Lines[0].AmountInCents = 99_00;`
+
+Acceptance: the **only** remaining build errors are the two inbound mappers.
+
+## Step 2 — reconstruct instead of write
+
+A DTO (and a storage row) carries one description and one flat total, so the
+reconstructed order is an order with **one line** — derived data cannot be
+written, only re-derived. In both `Mappers.FromOrderDto` (`UnmapOrder`) and
+`Mappers.FromOrderData`: keep the field copies as they are, drop the
+`TotalInCents = ...` assignment, and after creating the order add
+
+```csharp
+order.Lines.Add(new OrderLine(dto.Description, dto.TotalInCents));   // dto / row respectively
+```
+
+(always exactly one line — even for a zero total — so the reconstruction is
+deterministic). `DbContext.LoadOrder` / `LoadClient` need no changes.
+
+Acceptance: the full suite is green again, and `git diff --stat` shows the
+change confined to the domain, the builder, that one test, and the two
+inbound mapper bodies — `OrderDto`, `OrderData`, `DbContext`, `WebApp`, and
+the outbound mappers are untouched.
+
+## Step 3 — prove it: discount a reloaded order, and document the loss
+
+Create `tests/BeforeAfter.Tests/OrderLinesEvolutionTests.cs` from the stub
+below and make both tests pass:
+
+```csharp
+using After;
+using static BeforeAfter.Tests.TestData.ClientBuilder;
+using static BeforeAfter.Tests.TestData.OrderBuilder;
+
+namespace BeforeAfter.Tests;
+
+/// <summary>
+/// EXERCISE 2 — the domain gained OrderLines; persistence still stores one
+/// flat row per order. Make these tests pass (see EXERCISE2.md).
+/// </summary>
+public class OrderLinesEvolutionTests
+{
+    [Fact] // EXERCISE STEP 3a
+    public void A_reloaded_order_can_be_discounted()
+    {
+        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);
+
+        Order loaded = null!;   // TODO (step 3a): load the order back from the context
+
+        // The discount must work off the computed total of the RELOADED order:
+        var discounted = DiscountCalculator.CalculateDiscount(
+            loaded, discountBarrierCents: 100_00, discountPercentage: 10);
+
+        Assert.Equal(135_00, discounted.DiscountedTotalInCents);
+        Assert.Equal(10, discounted.DiscountPercentage);
+    }
+
+    [Fact] // EXERCISE STEP 3b — the accepted loss
+    public void A_multi_line_order_flattens_to_a_single_line_when_stored()
+    {
+        var db = new DbContext();
+        var order = ValidOrder().WithDescription("mixed order").Build();
+        order.Lines.Add(new OrderLine("first item", 100_00));
+        order.Lines.Add(new OrderLine("second item", 50_00));
+
+        db.Save(order);
+
+        Order loaded = null!;   // TODO (step 3b): load the order back from the context
+
+        // Storage flattened the order: one line, the flat total, and the
+        // order-level description. Document this — don't hide it.
+        var line = Assert.Single(loaded.Lines);
+        Assert.Equal(150_00, line.AmountInCents);
+        Assert.Equal("mixed order", line.Description);
+    }
+}
+```
+
+Acceptance: the whole suite is green — including Exercise 1's tests
+(`DiscountedOrderMappingTests`), which keep passing unchanged: the discount
+flow reads the computed total exactly as it read the stored one.
+
+## Facilitator notes
+
+- Trainer's solutions live on the `exercise-1-solution` and
+  `exercise-2-solution` branches — keep them off the participant path.
+
+- Expected diff per step: (1) `OrderLine` + `Order.Lines` + computed total +
+  builder internals + one test line — build red in exactly the two inbound
+  mappers; (2) two one-line reconstructions — green again; (3) two new tests.
+  If `git diff` touches `OrderDto`, `OrderData`, `DbContext`, `WebApp`, or the
+  outbound mappers, something went off the small-steps path.
+- The `WithTotal` pain the builder absorbs: internally it now fabricates a
+  line; no test noticed. That is the TestDataBuilder dividend.
+- Discussion points: what the next evolution would look like with a real
+  database (`order_line` rows and a foreign key — the persistence contract
+  would finally change, and only then); why the DTO cannot carry lines back
+  into the domain honestly (derived data is re-derived on the way in, never
+  transported); what the user of a reloaded multi-line order actually gets
+  (the flat total under the order's description) and why that must stay away
+  from the front-end.
+- `Before` stays untouched throughout — its Active Record world would have
+  needed a schema change on day one, which is the contrast this exercise
+  series is built on.

tests/BeforeAfter.Tests/AfterPersistenceTests.cs

@@ -1,4 +1,6 @@
 using After;
+using static BeforeAfter.Tests.TestData.ClientBuilder;
+using static BeforeAfter.Tests.TestData.OrderBuilder;
 
 namespace BeforeAfter.Tests;
 
@@ -15,9 +17,8 @@ public class AfterPersistenceTests
     public void Save_maps_domain_state_into_data_records()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
+        var order = ValidOrder().WithDescription("one").WithTotal(150_00).Build();
-        var order = new Order { Description = "one", TotalInCents = 15000 };
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
-        client.Orders.Add(order);
 
         db.Save(client);
         db.Save(order);
@@ -29,7 +30,7 @@ public class AfterPersistenceTests
         var orderRow = Assert.Single(db.Orders);
         Assert.Equal(order.Id, orderRow.Id);
         Assert.Equal("one", orderRow.Description);
-        Assert.Equal(15000, orderRow.TotalInCents);
+        Assert.Equal(150_00, orderRow.TotalInCents);
         Assert.Equal(client.Id, orderRow.ClientId);   // the graph is flattened to a key
     }
 
@@ -37,7 +38,7 @@ public class AfterPersistenceTests
     public void An_order_without_client_stores_an_empty_client_id()
     {
         var db = new DbContext();
-        var order = new Order { Description = "orphan" };
+        var order = ValidOrder().WithDescription("orphan").Build();
 
         db.Save(order);
 
@@ -48,14 +49,13 @@ public class AfterPersistenceTests
     public void Saving_again_refreshes_the_row_instead_of_duplicating_it()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
+        var order = ValidOrder().WithDescription("one").WithTotal(150_00).Build();
-        var order = new Order { Description = "one" };
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
-        client.Orders.Add(order);
         db.Save(client);
         db.Save(order);
 
         order.Description = "one (edited)";
-        order.TotalInCents = 9900;
+        order.TotalInCents = 99_00;
         client.Name = "Acme GmbH";
         db.Save(order);
         db.Save(client);
@@ -64,7 +64,7 @@ public class AfterPersistenceTests
         var orderRow = Assert.Single(db.Orders);
         Assert.Equal(order.Id, orderRow.Id);
         Assert.Equal("one (edited)", orderRow.Description);
-        Assert.Equal(9900, orderRow.TotalInCents);
+        Assert.Equal(99_00, orderRow.TotalInCents);
         Assert.Equal(client.Id, orderRow.ClientId);
 
         var clientRow = Assert.Single(db.Clients);
@@ -75,9 +75,8 @@ public class AfterPersistenceTests
     public void Rows_are_reachable_through_Set_and_Find()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
+        var order = ValidOrder().WithDescription("one").WithTotal(150_00).Build();
-        var order = new Order { Description = "one" };
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
-        client.Orders.Add(order);
         db.Save(client);
         db.Save(order);
 
@@ -98,9 +97,8 @@ public class AfterPersistenceTests
     public void Loading_an_order_restores_its_state_and_its_client()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
+        var order = ValidOrder().WithDescription("one").WithTotal(150_00).Build();
-        var order = new Order { Description = "one", TotalInCents = 150_00 };
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
-        client.Orders.Add(order);
         db.Save(client);
         db.Save(order);
 
@@ -122,12 +120,12 @@ public class AfterPersistenceTests
     public void Loading_a_client_restores_it_with_all_of_its_orders()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
+        var first = ValidOrder().WithDescription("one").WithTotal(150_00).Build();
-        client.Orders.Add(new Order { Description = "one", TotalInCents = 150_00 });
+        var second = ValidOrder().WithDescription("two").WithTotal(250_00).Build();
-        client.Orders.Add(new Order { Description = "two", TotalInCents = 250_00 });
+        var client = ValidClient().WithName("Acme").WithOrder(first).WithOrder(second).Build();
         db.Save(client);
-        db.Save(client.Orders[0]);
+        db.Save(first);
-        db.Save(client.Orders[1]);
+        db.Save(second);
 
         var loaded = db.LoadClient(client.Id);
 

tests/BeforeAfter.Tests/AfterTests.cs

@@ -1,4 +1,6 @@
 using After;
+using static BeforeAfter.Tests.TestData.ClientBuilder;
+using static BeforeAfter.Tests.TestData.OrderBuilder;
 using System.Reflection;
 
 namespace BeforeAfter.Tests;
@@ -7,6 +9,11 @@ namespace BeforeAfter.Tests;
 /// Tests for the AFTER situation (Yak 02): plain domain objects (no
 /// <see cref="DbBase"/>), DTOs (records) carrying the data including the
 /// relations, hand-written mappers, and a WebApp that consumes only DTOs.
+///
+/// Domain objects in these tests are constructed through the TestDataBuilders
+/// (<see cref="OrderBuilder.ValidOrder"/> / <see cref="ClientBuilder.ValidClient"/>)
+/// so the tests survive domain evolution; DTO initializers stay literal —
+/// DTOs are the transport shapes under test.
 /// </summary>
 public class AfterTests
 {
@@ -42,7 +49,7 @@ public class AfterTests
     public void ToClientDto_maps_all_scalar_fields()
     {
         var id = Guid.NewGuid();
-        var client = new Client { Id = id, Name = "Acme" };
+        var client = ValidClient().WithId(id).WithName("Acme").Build();
 
         var dto = Mappers.ToClientDto(client);
 
@@ -54,13 +61,13 @@ public class AfterTests
     public void ToOrderDto_maps_all_scalar_fields()
     {
         var id = Guid.NewGuid();
-        var order = new Order { Id = id, Description = "one", TotalInCents = 15000 };
+        var order = ValidOrder().WithId(id).WithDescription("one").WithTotal(150_00).Build();
 
         var dto = Mappers.ToOrderDto(order);
 
         Assert.Equal(id, dto.Id);
         Assert.Equal("one", dto.Description);
-        Assert.Equal(15000, dto.TotalInCents);
+        Assert.Equal(150_00, dto.TotalInCents);
     }
 
     [Fact]
@@ -79,13 +86,13 @@ public class AfterTests
     public void FromOrderDto_maps_all_scalar_fields()
     {
         var id = Guid.NewGuid();
-        var dto = new OrderDto { Id = id, Description = "one", TotalInCents = 15000 };
+        var dto = new OrderDto { Id = id, Description = "one", TotalInCents = 150_00 };
 
         var order = Mappers.FromOrderDto(dto);
 
         Assert.Equal(id, order.Id);
         Assert.Equal("one", order.Description);
-        Assert.Equal(15000, order.TotalInCents);
+        Assert.Equal(150_00, order.TotalInCents);
     }
 
     // ---------------------------------------------------------------------
@@ -95,9 +102,8 @@ public class AfterTests
     [Fact]
     public void ToClientDto_maps_orders_and_keeps_the_back_reference_consistent()
     {
-        var client = new Client { Name = "Acme" };
+        var order = ValidOrder().WithDescription("one").Build();
-        var order = new Order { Description = "one" };
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
-        client.Orders.Add(order);
 
         var dto = Mappers.ToClientDto(client);
         var orderDto = Assert.Single(dto.Orders);
@@ -130,9 +136,8 @@ public class AfterTests
     [Fact]
     public void Domain_round_trip_preserves_all_state()
     {
-        var client = new Client { Id = Guid.NewGuid(), Name = "Acme" };
+        var order = ValidOrder().WithId(Guid.NewGuid()).WithDescription("one").WithTotal(150_00).Build();
-        var order = new Order { Id = Guid.NewGuid(), Description = "one", TotalInCents = 15000 };
+        var client = ValidClient().WithId(Guid.NewGuid()).WithName("Acme").WithOrder(order).Build();
-        client.Orders.Add(order);
 
         var roundTrip = Mappers.FromClientDto(Mappers.ToClientDto(client));
 
@@ -149,9 +154,8 @@ public class AfterTests
     [Fact]
     public void Dto_round_trip_preserves_all_state()
     {
-        var client = new Client { Id = Guid.NewGuid(), Name = "Acme" };
+        var order = ValidOrder().WithId(Guid.NewGuid()).WithDescription("one").WithTotal(150_00).Build();
-        var order = new Order { Id = Guid.NewGuid(), Description = "one" };
+        var client = ValidClient().WithId(Guid.NewGuid()).WithName("Acme").WithOrder(order).Build();
-        client.Orders.Add(order);
         var dto = Mappers.ToClientDto(client);
         var dtoOrder = dto.Orders[0];
 
@@ -172,9 +176,8 @@ public class AfterTests
     [Fact]
     public void ToClientDto_does_not_mutate_the_domain()
     {
-        var client = new Client { Id = Guid.NewGuid(), Name = "Acme" };
+        var order = ValidOrder().WithId(Guid.NewGuid()).WithDescription("one").Build();
-        var order = new Order { Id = Guid.NewGuid(), Description = "one" };
+        var client = ValidClient().WithId(Guid.NewGuid()).WithName("Acme").WithOrder(order).Build();
-        client.Orders.Add(order);
         var ordersCount = client.Orders.Count;
 
         Mappers.ToClientDto(client);
@@ -247,9 +250,8 @@ public class AfterTests
     public void After_WebApp_consumes_dtos()
     {
         var app = new WebApp();
-        var client = new Client { Id = Guid.NewGuid(), Name = "Acme" };
+        var order = ValidOrder().WithId(Guid.NewGuid()).WithDescription("order one").WithTotal(150_00).Build();
-        var order = new Order { Id = Guid.NewGuid(), Description = "order one", TotalInCents = 15000 };
+        var client = ValidClient().WithId(Guid.NewGuid()).WithName("Acme").WithOrder(order).Build();
-        client.Orders.Add(order);
 
         var view = app.ShowClient(Mappers.ToClientDto(client));
 
@@ -266,9 +268,8 @@ public class AfterTests
     public void DbContext_saves_plain_domain_objects_and_tracks_them()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
+        var order = ValidOrder().WithDescription("one").Build();
-        var order = new Order { Description = "one" };
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
-        client.Orders.Add(order);
 
         Assert.Equal(Guid.Empty, client.Id);
         db.Save(client);
@@ -286,7 +287,7 @@ public class AfterTests
     public void Saving_twice_does_not_reassign_the_id()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
+        var client = ValidClient().WithName("Acme").Build();
         db.Save(client);
         var id = client.Id;
 
@@ -300,7 +301,7 @@ public class AfterTests
     public void A_saved_domain_object_keeps_no_reference_to_the_context()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
+        var client = ValidClient().WithName("Acme").Build();
         db.Save(client);
 
         // Saved (the id was assigned by the context) ...

tests/BeforeAfter.Tests/DiscountCalculatorTests.cs

@@ -1,4 +1,6 @@
 using After;
+using static BeforeAfter.Tests.TestData.ClientBuilder;
+using static BeforeAfter.Tests.TestData.OrderBuilder;
 
 namespace BeforeAfter.Tests;
 
@@ -16,7 +18,7 @@ public class DiscountCalculatorTests
     [Fact]
     public void A_total_below_the_barrier_gets_no_discount()
     {
-        var order = new Order { Id = Guid.NewGuid(), Description = "small", TotalInCents = 50_00 };
+        var order = ValidOrder().WithTotal(50_00).Build();
 
         var discounted = DiscountCalculator.CalculateDiscount(order, discountBarrierCents: 100_00, discountPercentage: 10);
 
@@ -28,7 +30,7 @@ public class DiscountCalculatorTests
     [Fact]
     public void A_total_exactly_at_the_barrier_gets_no_discount()
     {
-        var order = new Order { Id = Guid.NewGuid(), TotalInCents = 100_00 };
+        var order = ValidOrder().WithTotal(100_00).Build();
 
         var discounted = DiscountCalculator.CalculateDiscount(order, discountBarrierCents: 100_00, discountPercentage: 10);
 
@@ -42,7 +44,7 @@ public class DiscountCalculatorTests
     [Fact]
     public void A_total_above_the_barrier_gets_the_percentage_discount()
     {
-        var order = new Order { Id = Guid.NewGuid(), TotalInCents = 150_00 };
+        var order = ValidOrder().WithTotal(150_00).Build();
 
         var discounted = DiscountCalculator.CalculateDiscount(order, discountBarrierCents: 100_00, discountPercentage: 10);
 
@@ -54,7 +56,7 @@ public class DiscountCalculatorTests
     [Fact]
     public void A_zero_percentage_discounts_nothing_even_above_the_barrier()
     {
-        var order = new Order { Id = Guid.NewGuid(), TotalInCents = 150_00 };
+        var order = ValidOrder().WithTotal(150_00).Build();
 
         var discounted = DiscountCalculator.CalculateDiscount(order, discountBarrierCents: 100_00, discountPercentage: 0);
 
@@ -65,7 +67,7 @@ public class DiscountCalculatorTests
     [Fact]
     public void A_hundred_percent_discount_leaves_nothing()
     {
-        var order = new Order { Id = Guid.NewGuid(), TotalInCents = 150_00 };
+        var order = ValidOrder().WithTotal(150_00).Build();
 
         var discounted = DiscountCalculator.CalculateDiscount(order, discountBarrierCents: 100_00, discountPercentage: 100);
 
@@ -83,7 +85,7 @@ public class DiscountCalculatorTests
     [InlineData(151, 15, 136)]   // 15.1 rounds down to 15
     public void The_discount_amount_is_rounded_half_up(int totalInCents, int expectedDiscountCents, int expectedDiscountedTotal)
     {
-        var order = new Order { Id = Guid.NewGuid(), TotalInCents = totalInCents };
+        var order = ValidOrder().WithTotal(totalInCents).Build();
 
         var discounted = DiscountCalculator.CalculateDiscount(order, discountBarrierCents: 100, discountPercentage: 10);
 
@@ -98,7 +100,7 @@ public class DiscountCalculatorTests
     public void The_result_is_correlated_to_the_order_by_id()
     {
         var id = Guid.NewGuid();
-        var order = new Order { Id = id, TotalInCents = 150_00 };
+        var order = ValidOrder().WithId(id).WithTotal(150_00).Build();
 
         var discounted = DiscountCalculator.CalculateDiscount(order, discountBarrierCents: 100_00, discountPercentage: 10);
 
@@ -108,7 +110,7 @@ public class DiscountCalculatorTests
     [Fact]
     public void Calculating_does_not_mutate_the_order()
     {
-        var order = new Order { Id = Guid.NewGuid(), Description = "one", TotalInCents = 150_00 };
+        var order = ValidOrder().WithDescription("one").WithTotal(150_00).Build();
 
         DiscountCalculator.CalculateDiscount(order, discountBarrierCents: 100_00, discountPercentage: 10);
 
@@ -126,7 +128,7 @@ public class DiscountCalculatorTests
     [InlineData(150_00, 100_00, 101)]// percentage above 100
     public void Invalid_arguments_throw(int totalInCents, int discountBarrierCents, int discountPercentage)
     {
-        var order = new Order { Id = Guid.NewGuid(), TotalInCents = totalInCents };
+        var order = ValidOrder().WithTotal(totalInCents).Build();
 
         Assert.Throws<ArgumentOutOfRangeException>(
             () => DiscountCalculator.CalculateDiscount(order, discountBarrierCents, discountPercentage));

tests/BeforeAfter.Tests/TestData/ClientBuilder.cs

@@ -0,0 +1,40 @@
+using After;
+
+namespace BeforeAfter.Tests.TestData;
+
+/// <summary>
+/// Test data builder for a <see cref="Client"/> with its orders
+/// (TestDataBuilder pattern). <see cref="ValidClient"/> starts from a
+/// <em>fresh</em> client, exactly what <c>new Client()</c> gives.
+/// </summary>
+public sealed class ClientBuilder
+{
+    private readonly Client _client = new();
+
+    /// <summary>A valid, fresh client — the neutral starting point for tests.</summary>
+    public static ClientBuilder ValidClient() => new();
+
+    public ClientBuilder WithId(Guid id)
+    {
+        _client.Id = id;
+        return this;
+    }
+
+    public ClientBuilder WithName(string name)
+    {
+        _client.Name = name;
+        return this;
+    }
+
+    /// <summary>Adds an order to the client (navigation fix-up wires the back-reference).</summary>
+    public ClientBuilder WithOrder(Order order)
+    {
+        _client.Orders.Add(order);
+        return this;
+    }
+
+    /// <summary>Builds the given builder and adds its order — chaining sugar for <c>.WithOrder(builder.Build())</c>.</summary>
+    public ClientBuilder WithOrder(OrderBuilder order) => WithOrder(order.Build());
+
+    public Client Build() => _client;
+}

tests/BeforeAfter.Tests/TestData/OrderBuilder.cs

@@ -0,0 +1,47 @@
+using After;
+
+namespace BeforeAfter.Tests.TestData;
+
+/// <summary>
+/// Test data builder for an <see cref="Order"/> (TestDataBuilder pattern).
+/// <see cref="ValidOrder"/> starts from a <em>fresh</em> order —
+/// <see cref="Guid.Empty"/> id, empty description, zero total, exactly what
+/// <c>new Order()</c> gives — and every <c>With...</c> returns the builder for
+/// chaining; <see cref="Build"/> hands back the order.
+///
+/// This class is the only place tests touch an order's *construction*: when
+/// the domain evolves (e.g. OrderLines making the total computed), only these
+/// internals change and the tests keep reading the same.
+/// </summary>
+public sealed class OrderBuilder
+{
+    private readonly Order _order = new();
+
+    /// <summary>A valid, fresh order — the neutral starting point for tests.</summary>
+    public static OrderBuilder ValidOrder() => new();
+
+    public OrderBuilder WithId(Guid id)
+    {
+        _order.Id = id;
+        return this;
+    }
+
+    public OrderBuilder WithDescription(string description)
+    {
+        _order.Description = description;
+        return this;
+    }
+
+    /// <summary>
+    /// Sets the total in cents (<c>150_00</c> = €150.00). How the requested
+    /// total is realised inside the order is an implementation detail that may
+    /// change as the domain evolves; this API stays.
+    /// </summary>
+    public OrderBuilder WithTotal(int totalInCents)
+    {
+        _order.TotalInCents = totalInCents;
+        return this;
+    }
+
+    public Order Build() => _order;
+}