refactor: remove Source property from IRelationship

Remove the redundant `Source` property from `IRelationship` and its
implementations. Since the relationship component is inherently
stored on the source entity, the `Source` field was unnecessary and
required manual synchronization during deserialization.

This change simplifies the relationship model, reduces memory usage,
and removes the need for reflection-based fixups in the serializer.

Game.Blackjack.Tests/GameFlowTests.cs

@@ -243,7 +243,7 @@ public class GameFlowTests
 
     private static int CountCardsInHand(World world, Entity handEntity)
     {
-        // Holds relationship: Source = card entity, Target = hand entity.
+        // Holds relationship: the card entity (which Holds points to the hand entity).
         // GetSources returns all card entities that have a Holds pointing to handEntity.
         return world.GetSources<Holds>(handEntity).Count;
     }

Game.Blackjack/Commands/HitCommand.cs

@@ -41,7 +41,7 @@ public struct HitCommand : ICommand
 
         // Remove from deck, add to hand.
         world.RemoveComponent<InDeck>(cardEntity);
-        world.AddComponent(cardEntity, new Holds { Source = cardEntity, Target = handEntity });
+        world.AddComponent(cardEntity, new Holds { Target = handEntity });
     }
 
     internal static Entity FindEntity<T>(World world, Entity singletonEntity)

Game.Blackjack/Commands/NewRoundCommand.cs

@@ -42,7 +42,7 @@ public struct NewRoundCommand : ICommand
             foreach (var card in cards)
             {
                 world.RemoveComponent<Holds>(card);
-                world.AddComponent(card, new InDeck { Source = card, Target = deckEntity });
+                world.AddComponent(card, new InDeck { Target = deckEntity });
             }
         }
 

Game.Blackjack/Components/Holds.cs

@@ -10,6 +10,5 @@ namespace Game.Blackjack;
 [MessagePackObject]
 public record struct Holds : IRelationship
 {
-    [Key(0)] public Entity Source { get; set; }
+    [Key(0)] public Entity Target { get; set; }
-    [Key(1)] public Entity Target { get; set; }
 }

Game.Blackjack/Components/InDeck.cs

@@ -10,6 +10,5 @@ namespace Game.Blackjack;
 [MessagePackObject]
 public record struct InDeck : IRelationship
 {
-    [Key(0)] public Entity Source { get; set; }
+    [Key(0)] public Entity Target { get; set; }
-    [Key(1)] public Entity Target { get; set; }
 }

Game.Blackjack/Systems/DeckSetupSystem.cs

@@ -36,7 +36,7 @@ public class DeckSetupSystem : ISystem
                 {
                     var cardEntity = world.CreateEntity();
                     world.AddComponent(cardEntity, new Card { Suit = suit, Rank = rank });
-                    world.AddComponent(cardEntity, new InDeck { Source = cardEntity, Target = deckEntity });
+                    world.AddComponent(cardEntity, new InDeck { Target = deckEntity });
                 }
             }
 
@@ -81,7 +81,7 @@ public class DeckSetupSystem : ISystem
         }
         for (int i = 0; i < cardEntities.Length; i++)
         {
-            world.AddComponent(cardEntities[i], new InDeck { Source = cardEntities[i], Target = deckEntity });
+            world.AddComponent(cardEntities[i], new InDeck { Target = deckEntity });
         }
     }
 }

Game.CardWars/CardEffects.cs

@@ -206,7 +206,7 @@ public class NunEffect : ICardEffect
             world.RemoveComponent<OnField>(target);
             if (world.HasComponent<Card>(target))
                 world.RemoveComponent<Card>(target);
-            world.AddComponent(target, new InDeck { Source = target, Target = discardDeck });
+            world.AddComponent(target, new InDeck { Target = discardDeck });
         }
         world.Commands.Enqueue(new DrawCardCommand { Player = pending.Player, Kind = null });
         world.RemoveSingleton<PendingNun>();
@@ -293,7 +293,7 @@ public class PegasusEffect : ICardEffect
             world.RemoveComponent<OnField>(target);
             if (world.HasComponent<Card>(target))
                 world.RemoveComponent<Card>(target);
-            world.AddComponent(target, new HeldBy { Source = target, Target = pending.Player });
+            world.AddComponent(target, new HeldBy { Target = pending.Player });
         }
         world.RemoveSingleton<PendingPegasus>();
         return true;
@@ -321,7 +321,7 @@ public class CurseMasterEffect : ICardEffect
             {
                 var skull = world.CreateEntity();
                 world.AddComponent(skull, new Skull());
-                world.AddComponent(skull, new PlacedOn { Source = skull, Target = pt });
+                world.AddComponent(skull, new PlacedOn { Target = pt });
             }
         }
         world.RemoveSingleton<PendingCurseMaster>();
@@ -366,14 +366,14 @@ public static class CardEffectHelpers
     {
         var horn = world.CreateEntity();
         world.AddComponent(horn, new Horn());
-        world.AddComponent(horn, new PlacedOn { Source = horn, Target = target });
+        world.AddComponent(horn, new PlacedOn { Target = target });
     }
 
     public static void AddSkull(World world, Entity target)
     {
         var skull = world.CreateEntity();
         world.AddComponent(skull, new Skull());
-        world.AddComponent(skull, new PlacedOn { Source = skull, Target = target });
+        world.AddComponent(skull, new PlacedOn { Target = target });
     }
 
     public static Entity GetBanner(World world, Entity player)

Game.CardWars/Commands/PlayCardCommand.cs

@@ -38,7 +38,7 @@ public struct PlayCardCommand : ICommand
 
         // Move from hand to field.
         world.RemoveComponent<HeldBy>(CardEntity);
-        world.AddComponent(CardEntity, new OnField { Source = CardEntity, Target = fieldOwner });
+        world.AddComponent(CardEntity, new OnField { Target = fieldOwner });
         world.AddComponent(CardEntity, new Card { Rank = Rank, FaceDown = FaceDown });
 
         // Resolve on-play effects via registry.

Game.CardWars/Components/HeldBy.cs

@@ -10,6 +10,5 @@ namespace Game.CardWars;
 [MessagePackObject]
 public record struct HeldBy : IRelationship
 {
-    [Key(0)] public Entity Source { get; set; }
+    [Key(0)] public Entity Target { get; set; }
-    [Key(1)] public Entity Target { get; set; }
 }

Game.CardWars/Components/InDeck.cs

@@ -9,6 +9,5 @@ namespace Game.CardWars;
 [MessagePackObject]
 public record struct InDeck : IRelationship
 {
-    [Key(0)] public Entity Source { get; set; }
+    [Key(0)] public Entity Target { get; set; }
-    [Key(1)] public Entity Target { get; set; }
 }

Game.CardWars/Components/OnField.cs

@@ -9,6 +9,5 @@ namespace Game.CardWars;
 [MessagePackObject]
 public record struct OnField : IRelationship
 {
-    [Key(0)] public Entity Source { get; set; }
+    [Key(0)] public Entity Target { get; set; }
-    [Key(1)] public Entity Target { get; set; }
 }

Game.CardWars/Components/PlacedOn.cs

@@ -9,6 +9,5 @@ namespace Game.CardWars;
 [MessagePackObject]
 public record struct PlacedOn : IRelationship
 {
-    [Key(0)] public Entity Source { get; set; }
+    [Key(0)] public Entity Target { get; set; }
-    [Key(1)] public Entity Target { get; set; }
 }

Game.CardWars/GameUtil.cs

@@ -38,7 +38,7 @@ public static class GameUtil
 
         var cardEntity = cards.First();
         world.RemoveComponent<InDeck>(cardEntity);
-        world.AddComponent(cardEntity, new HeldBy { Source = cardEntity, Target = player });
+        world.AddComponent(cardEntity, new HeldBy { Target = player });
         return true;
     }
 
@@ -54,7 +54,7 @@ public static class GameUtil
         for (int i = cards.Length - 1; i >= 0; i--)
             world.RemoveComponent<InDeck>(cards[i]);
         for (int i = 0; i < cards.Length; i++)
-            world.AddComponent(cards[i], new InDeck { Source = cards[i], Target = deck });
+            world.AddComponent(cards[i], new InDeck { Target = deck });
     }
 
     /// <summary>Get all field cards for a player.</summary>
@@ -135,7 +135,7 @@ public static class GameUtil
         {
             world.RemoveComponent<OnField>(card);
             world.RemoveComponent<Card>(card);
-            world.AddComponent(card, new InDeck { Source = card, Target = discardDeck });
+            world.AddComponent(card, new InDeck { Target = discardDeck });
         }
 
         // Destroy horns and skulls placed on discarded cards.
@@ -156,7 +156,7 @@ public static class GameUtil
         foreach (var card in cards)
         {
             world.RemoveComponent<InDeck>(card);
-            world.AddComponent(card, new InDeck { Source = card, Target = drawDeck });
+            world.AddComponent(card, new InDeck { Target = drawDeck });
         }
     }
 }

Game.CardWars/Systems/GameSetupSystem.cs

@@ -57,7 +57,7 @@ public class GameSetupSystem : ISystem
         var defs = CardDataLoader.LoadDefinitions(world, _cardDataCsv);
         foreach (var (defEntity, _) in defs)
         {
-            world.AddComponent(defEntity, new InDeck { Source = defEntity, Target = publicDeck });
+            world.AddComponent(defEntity, new InDeck { Target = publicDeck });
         }
 
         // Shuffle public deck.
@@ -79,12 +79,12 @@ public class GameSetupSystem : ISystem
             world.AddComponent(leader, new Leader());
             world.AddComponent(leader, new CardDef { Name = $"领袖{i}", Ranks = new[] { 0 }, Effect = CardEffect.None, Kind = CardKind.Faction });
             world.AddComponent(leader, new Card { Rank = 0, FaceDown = false });
-            world.AddComponent(leader, new OnField { Source = leader, Target = player });
+            world.AddComponent(leader, new OnField { Target = player });
 
             // Create banner.
             var banner = world.CreateEntity();
             world.AddComponent(banner, new Banner());
-            world.AddComponent(banner, new OnField { Source = banner, Target = player });
+            world.AddComponent(banner, new OnField { Target = player });
 
             // Create faction deck.
             var factionDeck = world.CreateEntity();

Game.CardWars/Systems/ScoringSystem.cs

@@ -42,7 +42,7 @@ public class ScoringSystem : ISystem
             if (castles.Count > 0)
             {
                 var castle = castles[0];
-                world.AddComponent(castle, new HeldBy { Source = castle, Target = winner });
+                world.AddComponent(castle, new HeldBy { Target = winner });
                 world.RemoveComponent<Castle>(castle);
             }
 

OECS.Tests/EdgeCaseTests.cs

@@ -111,7 +111,6 @@ public class EdgeCaseTests
             sources.Add(source);
             world.AddComponent(source, new Relationship<ChildOf, ParentOf>
             {
-                Source = source,
                 Target = target
             });
         }

OECS.Tests/RelationshipTests.cs

@@ -24,7 +24,6 @@ public class RelationshipTests
 
         world.AddComponent(child, new Relationship<ChildOf, ParentOf>
         {
-            Source = child,
             Target = parent
         });
 
@@ -41,7 +40,6 @@ public class RelationshipTests
 
         world.AddComponent(child, new Relationship<ChildOf, ParentOf>
         {
-            Source = child,
             Target = parent
         });
 
@@ -61,14 +59,12 @@ public class RelationshipTests
 
         world.AddComponent(child, new Relationship<ChildOf, ParentOf>
         {
-            Source = child,
             Target = parent1
         });
 
         // Replace with a new target.
         world.AddComponent(child, new Relationship<ChildOf, ParentOf>
         {
-            Source = child,
             Target = parent2
         });
 
@@ -85,7 +81,6 @@ public class RelationshipTests
 
         world.AddComponent(child, new Relationship<ChildOf, ParentOf>
         {
-            Source = child,
             Target = parent
         });
 
@@ -107,7 +102,6 @@ public class RelationshipTests
 
         world.AddComponent(child, new Relationship<ChildOf, ParentOf>
         {
-            Source = child,
             Target = parent
         });
 
@@ -131,15 +125,15 @@ public class RelationshipTests
 
         world.AddComponent(child1, new Relationship<ChildOf, ParentOf>
         {
-            Source = child1, Target = parent
+            Target = parent
         });
         world.AddComponent(child2, new Relationship<ChildOf, ParentOf>
         {
-            Source = child2, Target = parent
+            Target = parent
         });
         world.AddComponent(child3, new Relationship<ChildOf, ParentOf>
         {
-            Source = child3, Target = parent
+            Target = parent
         });
 
         var sources = world.GetSources<Relationship<ChildOf, ParentOf>>(parent);
@@ -156,11 +150,11 @@ public class RelationshipTests
 
         world.AddComponent(entity, new Relationship<OwnedBy, Thing>
         {
-            Source = entity, Target = owner
+            Target = owner
         });
         world.AddComponent(entity, new Relationship<MemberOf, Thing>
         {
-            Source = entity, Target = group
+            Target = group
         });
 
         world.GetSources<Relationship<OwnedBy, Thing>>(owner).Should().BeEquivalentTo([entity]);
@@ -189,7 +183,7 @@ public class RelationshipTests
 
         world.AddComponent(child, new Relationship<ChildOf, ParentOf>
         {
-            Source = child, Target = parent
+            Target = parent
         });
 
         var query = world.Query().With<Relationship<ChildOf, ParentOf>>().Build();
@@ -215,13 +209,13 @@ public class RelationshipTests
         // a → b (a is child of b)
         world.AddComponent(a, new Relationship<ChildOf, ParentOf>
         {
-            Source = a, Target = b
+            Target = b
         });
 
         // c → a (c is child of a)
         world.AddComponent(c, new Relationship<ChildOf, ParentOf>
         {
-            Source = c, Target = a
+            Target = a
         });
 
         // Destroy a. This should:

OECS/IRelationship.cs

@@ -2,19 +2,14 @@ namespace OECS;
 
 /// <summary>
 /// Marker interface for components that represent a directed relationship
-/// between two entities. The component is stored on the <see cref="Source"/>
+/// between two entities. The component is stored on the source entity
-/// entity and points to the <see cref="Target"/> entity.
+/// (the entity it's added to) and points to the <see cref="Target"/> entity.
 ///
 /// The <see cref="World"/> automatically maintains a reverse index so that
 /// all sources pointing to a given target can be looked up efficiently.
 /// </summary>
 public interface IRelationship
 {
-    /// <summary>
-    /// The entity that owns this relationship component.
-    /// </summary>
-    Entity Source { get; }
-
     /// <summary>
     /// The entity that this relationship points to.
     /// </summary>

OECS/Relationship.cs

@@ -14,7 +14,6 @@ namespace OECS;
 /// // Define a "ChildOf" relationship between a child and a parent entity.
 /// world.AddComponent(child, new Relationship<ChildOf, Parent>
 /// {
-///     Source = child,
 ///     Target = parent
 /// });
 ///
@@ -27,15 +26,9 @@ namespace OECS;
 public struct Relationship<TSelf, TTarget> : IRelationship
     where TSelf : struct where TTarget : struct
 {
-    /// <summary>
-    /// The entity that owns this relationship component.
-    /// </summary>
-    [Key(0)]
-    public Entity Source { get; set; }
-
     /// <summary>
     /// The entity that this relationship points to.
     /// </summary>
-    [Key(1)]
+    [Key(0)]
     public Entity Target { get; set; }
 }

OECS/World.cs

@@ -203,19 +203,8 @@ public class World : IDisposable
         ThrowIfNotAlive(entity);
 
         // If replacing an existing relationship, remove the old index entry first.
-        if (component is IRelationship newRel)
+        if (component is IRelationship)
         {
-            // Guard against mismatched Source: the relationship must be stored
-            // on the entity it claims as its Source, otherwise the reverse
-            // index becomes corrupted.
-            if (newRel.Source != entity)
-            {
-                throw new InvalidOperationException(
-                    $"Relationship of type {typeof(T).Name} has Source={newRel.Source} " +
-                    $"but is being added to entity {entity}. " +
-                    $"The Source must match the entity the component is added to.");
-            }
-
             if (_components.TryGet<T>(entity, out var old))
             {
                 var oldRel = (IRelationship)(object)old;

OECS/WorldSerializer.cs

@@ -1,4 +1,3 @@
-using System.Reflection;
 using MessagePack;
 
 namespace OECS;
@@ -94,32 +93,10 @@ public static class WorldSerializer
                         $"Ensure the component type is used with the World " +
                         $"API so the source generator can discover it.");
 
-                // Deserialize and fix up IRelationship.Source before adding.
+                // Deserialize and add via the typed internal method.
                 var component = desc.Deserialize(ce.Data);
-                if (component is IRelationship rel)
-                    FixupRelationshipSource(rel, entity);
-
-                // Add via the typed internal method — no reflection for the add itself.
                 world.AddComponentBoxed(entity, component, desc.Type);
             }
         }
     }
-
-    /// <summary>
-    /// Sets the Source field on a deserialized IRelationship to match
-    /// the entity it's being restored to. Uses a small cache of PropertyInfo
-    /// to avoid repeated reflection lookups.
-    /// </summary>
-    private static void FixupRelationshipSource(IRelationship rel, Entity entity)
-    {
-        var type = rel.GetType();
-        if (!_sourcePropCache.TryGetValue(type, out var prop))
-        {
-            prop = type.GetProperty("Source");
-            _sourcePropCache[type] = prop;
-        }
-        prop?.SetValue(rel, entity);
-    }
-
-    private static readonly Dictionary<Type, PropertyInfo?> _sourcePropCache = new();
 }