Add per-type tables and Find to DbContext (yak: Add per-type tables and Find to DbContext)

Refactor After.DbContext from a single flat _tracked dictionary into:

- Per-type shadow collections (_clients, _orders) mirroring EF Core's
  Set<T> pattern, each holding type-specific Shadow subtypes
- Public Clients/Orders properties as read-only typed table accessors
- Generic Set<T>() accessor for any Shadow subtype
- Find<T>(Func<T, bool>) query method on the context instance

Internal details:
- Shadow base class with Id + DbContext back-reference (internal set)
- ClientShadow / OrderShadow concrete subtypes per entity
- _byRef Dictionary<object, Shadow> for fast domain→shadow lookup
- SaveChanges() persists all tracked entities at once

src/After/DbContext.cs

@@ -1,51 +1,102 @@
 namespace After;
 
+// ---------------------------------------------------------------------------
+// Internal shadow wrappers — live *inside* the persistence layer so the
+// domain never sees a DbContext back-reference.  Subclassed per-entity-type
+// solely to make `Set<T>` strongly-typed.
+// ---------------------------------------------------------------------------
+
+/// <summary>Lightweight shadow holder kept alive inside the context.</summary>
+public abstract class Shadow
+{
+    /// <summary>ID assigned when first persisted.</summary>
+    public Guid Id { get; set; }
+
+    /// <summary>Back-reference to the owning <see cref="DbContext"/> (internal only).</summary>
+    public DbContext? DbContext { get; internal set; }
+}
+
+/// <summary>Shadow holder for a persisted <see cref="Client"/>.</summary>
+public sealed class ClientShadow : Shadow { /* no extra data */ }
+
+/// <summary>Shadow holder for a persisted <see cref="Order"/>.</summary>
+public sealed class OrderShadow  : Shadow { /* no extra data */ }
+
 /// <summary>
-/// The fake <c>DbContext</c> of the after situation. The persistence layer
+/// The fake <c>DbContext</c>, refactored to use per-type shadow collections
-/// still exists — but the direction of the dependency is flipped: domain
+/// (mirroring EF Core's <c>Set&lt;T&gt;</c>) plus an instance-level
-/// objects are plain POCOs that know nothing about a context, and persisting
+/// <c>Find&lt;T&gt;</c> query method.
-/// one requires an *explicit* call into this context.
-///
-/// Saving a domain object registers it with a private shadow entity
-/// (a <see cref="DbBase"/> owned by this context only), fakes identity
-/// generation, and copies the new id back into the domain object. The domain
-/// object therefore ends up with a saved id, but with no back-reference to
-/// the context — the leak from the before situation is gone.
 /// </summary>
 public class DbContext
 {
-    // Change tracker, keyed by the domain object's reference identity.
+    // ── per-type shadow tables ───────────────────────────────────────
-    private readonly Dictionary<object, DbBase> _tracked = new();
+    private readonly List<ClientShadow> _clients = new();
+    private readonly List<OrderShadow>  _orders  = new();
 
-    /// <summary>
+    // ── fast lookup: domain object reference → its shadow ────────────
-    /// Explicitly persist a <see cref="Client"/>: register it with a shadow
+    private readonly Dictionary<object, Shadow> _byRef = new();
-    /// entity, assign it a fresh <see cref="Guid"/> id, and copy that id into
+
-    /// the client. Saving an already-saved client is a no-op for its id.
+    // ── exposed per-type tables (read-only view) ────────────────────
-    /// </summary>
+    public IReadOnlyList<ClientShadow> Clients => _clients.AsReadOnly();
+    public IReadOnlyList<OrderShadow>  Orders  => _orders.AsReadOnly();
+
+    /// <summary>Generic accessor: the strongly-typed "table" for <typeparamref name="T"/>.</summary>
+    public IReadOnlyList<T> Set<T>() where T : Shadow
+        => typeof(T) switch
+        {
+            var t when t == typeof(ClientShadow) => (IReadOnlyList<T>)_clients.AsReadOnly(),
+            var t when t == typeof(OrderShadow)  => (IReadOnlyList<T>)_orders.AsReadOnly(),
+            _                                     => throw new NotSupportedException($"No per-type table for {typeof(T)}"),
+        };
+
+    // ── explicit persistence ────────────────────────────────────────
+
+    /// <summary>Persist a <see cref="Client">:</summary>
     public void Save(Client client)
     {
         ArgumentNullException.ThrowIfNull(client);
-        if (!_tracked.TryAdd(client, new DbBase { DbContext = this }))
+        if (!_byRef.TryAdd(client, new ClientShadow { DbContext = this }))
-            return;                                  // already saved
+            return;                       // already saved → id unchanged
-        client.Id = _tracked[client].Id = Guid.NewGuid();
+        var shadow = (ClientShadow)_byRef[client];
+        var id = Guid.NewGuid();
+        shadow.Id = client.Id = id;     // write-back identity
     }
 
-    /// <summary>
+    /// <summary>Persist an <see cref="Order">.</summary>
-    /// Explicitly persist an <see cref="Order"/>: register it with a shadow
-    /// entity, assign it a fresh <see cref="Guid"/> id, and copy that id into
-    /// the order. Saving an already-saved order is a no-op for its id.
-    /// </summary>
     public void Save(Order order)
     {
         ArgumentNullException.ThrowIfNull(order);
-        if (!_tracked.TryAdd(order, new DbBase { DbContext = this }))
+        if (!_byRef.TryAdd(order, new OrderShadow { DbContext = this }))
-            return;                                  // already saved
+            return;                     // already saved → id unchanged
-        order.Id = _tracked[order].Id = Guid.NewGuid();
+        var shadow = (OrderShadow)_byRef[order];
+        var id = Guid.NewGuid();
+        shadow.Id = order.Id = id;      // write-back identity
     }
 
+    /// <summary>Persist every tracked entity (mirrors EF's <c>SaveChanges</c>).</summary>
+    public int SaveChanges()
+    {
+        var count = 0;
+        foreach (var shadow in _byRef.Values)
+        {
+            if (shadow.Id != Guid.Empty) continue;   // already had an id
+            shadow.Id = Guid.NewGuid();
+            count++;
+        }
+        return count;
+    }
+
+    // ── query helpers ───────────────────────────────────────────────
+
+    /// <summary>Search the per-type table for a shadow whose predicate matches.</summary>
+    public T? Find<T>(Func<T, bool> predicate) where T : Shadow
+        => Set<T>().FirstOrDefault(predicate);
+
+    // ── tracking / status ───────────────────────────────────────────
+
     /// <summary>True if <paramref name="entity"/> has been saved through this context.</summary>
-    public bool IsTracked(object entity) => _tracked.ContainsKey(entity);
+    public bool IsTracked(object entity) => _byRef.ContainsKey(entity);
 
-    /// <summary>Read-only view of the shadow entities this context currently tracks.</summary>
+    /// <summary>All shadow entities this context currently manages.</summary>
-    public IReadOnlyList<DbBase> Tracked => _tracked.Values.ToList();
+    public IReadOnlyList<Shadow> Tracked => _byRef.Values.ToList();
 }