persistence records carry data: ClientData/OrderData + domain->persistence mappers

The fake DbContext now stores data-only DbBase subclasses (renamed from
the identity-only Shadow wrappers) and refreshes rows in place on re-Save;
the object graph is flattened to ids in storage.

yak: data-carrying-persistence-records-orderdataclientdata-mappers-tb3a

src/After/Mappers.cs

@@ -1,13 +1,21 @@
 namespace After;
 
 /// <summary>
-/// Hand-written mappers between the plain domain and the DTOs (no AutoMapper —
-/// the mapping code is part of the illustration). Each public method maps in
-/// exactly one direction and never mutates its input.
+/// Hand-written mappers out of the plain domain (no AutoMapper — the mapping
+/// code is part of the illustration). Each public method maps in exactly one
+/// direction and never mutates its input. Two families, two destinations:
+/// <list type="bullet">
+/// <item><see cref="ClientDto"/>/<see cref="OrderDto"/> — transport to the
+/// WebApp;</item>
+/// <item><see cref="ClientData"/>/<see cref="OrderData"/> — the data-only
+/// records the persistence framework stores.</item>
+/// </list>
 ///
 /// The <c>Client.Orders</c> / <c>Order.Client</c> back-reference cycle is
 /// resolved with a reference-identity cache: the same instance maps to the
-/// same DTO (and back), so the graph terminates and stays consistent.
+/// same DTO (and back), so the graph terminates and stays consistent. The
+/// persistence mapping needs no cache: records reference each other by id,
+/// not by object, so the graph is flattened and cannot cycle.
 /// </summary>
 public static class Mappers
 {
@@ -99,4 +107,32 @@ public static class Mappers
         seen[dto] = order;
         return order;
     }
+
+    // ------------------------------------------------------------------
+    // Domain -> persistence (data-only records; graph flattened to ids)
+    // ------------------------------------------------------------------
+
+    /// <summary>Map a <see cref="Client"/> to its persistence record <see cref="ClientData"/>.</summary>
+    public static ClientData ToClientData(Client client)
+    {
+        ArgumentNullException.ThrowIfNull(client);
+        return new ClientData { Id = client.Id, Name = client.Name };
+    }
+
+    /// <summary>
+    /// Map an <see cref="Order"/> to its persistence record <see cref="OrderData"/>:
+    /// the object graph is flattened — <see cref="Order.Client"/> becomes the
+    /// <see cref="OrderData.ClientId"/> foreign key (empty when the order has
+    /// no client, or when that client has not been saved yet).
+    /// </summary>
+    public static OrderData ToOrderData(Order order)
+    {
+        ArgumentNullException.ThrowIfNull(order);
+        return new OrderData
+        {
+            Id = order.Id,
+            Description = order.Description,
+            ClientId = order.Client?.Id ?? Guid.Empty,
+        };
+    }
 }