make database access perform more realistically

.gitignore

@@ -7,6 +7,7 @@ obj/
 .vscode/
 *.user
 *.suo
+.idea
 
 ## Dotnet CLI
 .dotnet/

src/Before/DbContext.cs

@@ -18,6 +18,21 @@ public class DbContext
     // maintains a separate set per entity type.
     private readonly Dictionary<Type, Collection<DbBase>> _tables = new();
 
+    // -----------------------------------------------------------------
+    // Simulated DB latency. Every "round trip" through this context —
+    // Attach, Save, Find, IsTracked, Tracked — sleeps for this long, the
+    // way a real database makes every call pay a network + query cost.
+    // Deliberately on by default: the console demo and the tests are
+    // supposed to feel sluggish, so the cost of persistence becomes
+    // visible. Set to TimeSpan.Zero for an instant fake.
+    // -----------------------------------------------------------------
+    /// <summary>
+    /// Simulated latency per DB operation. Default 200 ms.
+    /// </summary>
+    public static TimeSpan Latency { get; set; } = TimeSpan.FromMilliseconds(200);
+
+    private static void SimulateLatency() => Thread.Sleep(Latency);
+
     /// <summary>
     /// Get or create the per-type table for <typeparamref name="T"/>.
     /// This mirrors EF's <see cref="DbSet{T}"/> / <c>Set&lt;T&gt;</c> accessor.
@@ -55,6 +70,7 @@ public class DbContext
     /// </summary>
     public void Attach(DbBase entity)
     {
+        SimulateLatency();
         ArgumentNullException.ThrowIfNull(entity);
         entity.DbContext = this;
         var table = TableFor(entity.GetType());
@@ -70,6 +86,7 @@ public class DbContext
     /// </summary>
     public int Save()
     {
+        SimulateLatency();
         var saved = 0;
         foreach (var table in _tables.Values)
             foreach (var entity in table)
@@ -95,6 +112,7 @@ public class DbContext
     /// <returns>The first matching entity, or <c>null</c> when no match.</returns>
     public T? Find<T>(Predicate<T> predicate) where T : DbBase
     {
+        SimulateLatency();
         var table = Table<T>();
         foreach (var e in table)
             if (e is T candidate && predicate(candidate))
@@ -108,6 +126,7 @@ public class DbContext
     /// </summary>
     public bool IsTracked(DbBase entity)
     {
+        SimulateLatency();
         foreach (var table in _tables.Values)
             if (table.Contains(entity))
                 return true;
@@ -122,6 +141,7 @@ public class DbContext
     {
         get
         {
+            SimulateLatency();
             var all = new List<DbBase>();
             foreach (var table in _tables.Values)
                 all.AddRange(table);