refactor: optimize query execution and improve change tracking

- Optimize `QueryExecutor` to use the smallest sparse set as a driver
  for multi-component queries, reducing iteration overhead.
- Implement automatic cleanup of unused `Subject` instances in
  `ChangeBuffer` using a subscriber count.
- Improve `ChangeSet` deduplication to correctly handle rapid
  add/remove transitions for the same component.
- Add validation to ensure `IRelationship` components are added to
  their declared `Source` entity.
- Update documentation to reflect correct `Entity` bit layout.

docs/architecture.md

@@ -45,8 +45,8 @@ signature) and sparse sets (one dense array per component type).
 **Context:** Entities need to be cheap to copy, comparable, and safe against
 use-after-free (accessing a recycled entity ID).
 
-**Decision:** `readonly struct Entity` wrapping a `uint`. Upper 24 bits are the
+**Decision:** `readonly struct Entity` wrapping a `uint`. Lower 24 bits are the
-ID, lower 8 bits are the version.
+ID, upper 8 bits are the version.
 
 **Rationale:**
 

src/OECS/ChangeBuffer.cs

@@ -14,8 +14,18 @@ internal class ChangeBuffer
     private readonly ChangeSet _pending = new();
 
     private readonly Subject<EntityChange> _entitySubject = new();
-    private readonly Dictionary<Type, Subject<EntityChange>> _componentSubjects = new();
+    private readonly Dictionary<Type, TrackedSubject> _componentSubjects = new();
-    private readonly Dictionary<QueryDescriptor, Subject<EntityChange>> _querySubjects = new();
+    private readonly Dictionary<QueryDescriptor, TrackedSubject> _querySubjects = new();
+
+    /// <summary>
+    /// Wraps a <see cref="Subject{T}"/> with a subscriber count so that
+    /// subjects with no remaining subscribers can be cleaned up.
+    /// </summary>
+    private sealed class TrackedSubject
+    {
+        public readonly Subject<EntityChange> Subject = new();
+        public int SubscriberCount;
+    }
 
     /// <summary>
     /// The change set currently accumulating. Cleared after each <see cref="Post"/>.
@@ -40,18 +50,18 @@ internal class ChangeBuffer
             // Push to component-specific subjects.
             if (change.ComponentType != null)
             {
-                if (_componentSubjects.TryGetValue(change.ComponentType, out var compSubject))
+                if (_componentSubjects.TryGetValue(change.ComponentType, out var compTracked))
                 {
-                    compSubject.OnNext(change);
+                    compTracked.Subject.OnNext(change);
                 }
             }
 
             // Push to matching query subjects.
-            foreach (var (query, subject) in _querySubjects)
+            foreach (var (query, tracked) in _querySubjects)
             {
                 if (ChangeMatchesQuery(change, query))
                 {
-                    subject.OnNext(change);
+                    tracked.Subject.OnNext(change);
                 }
             }
         }
@@ -72,12 +82,22 @@ internal class ChangeBuffer
     /// </summary>
     public Observable<EntityChange> ObserveComponentChanges(Type componentType)
     {
-        if (!_componentSubjects.TryGetValue(componentType, out var subject))
+        if (!_componentSubjects.TryGetValue(componentType, out var tracked))
         {
-            subject = new Subject<EntityChange>();
+            tracked = new TrackedSubject();
-            _componentSubjects[componentType] = subject;
+            _componentSubjects[componentType] = tracked;
         }
-        return subject;
+
+        tracked.SubscriberCount++;
+        return WrapWithCleanup(tracked.Subject, () =>
+        {
+            tracked.SubscriberCount--;
+            if (tracked.SubscriberCount <= 0)
+            {
+                tracked.Subject.Dispose();
+                _componentSubjects.Remove(componentType);
+            }
+        });
     }
 
     /// <summary>
@@ -85,12 +105,40 @@ internal class ChangeBuffer
     /// </summary>
     public Observable<EntityChange> ObserveQuery(QueryDescriptor query)
     {
-        if (!_querySubjects.TryGetValue(query, out var subject))
+        if (!_querySubjects.TryGetValue(query, out var tracked))
         {
-            subject = new Subject<EntityChange>();
+            tracked = new TrackedSubject();
-            _querySubjects[query] = subject;
+            _querySubjects[query] = tracked;
         }
-        return subject;
+
+        tracked.SubscriberCount++;
+        return WrapWithCleanup(tracked.Subject, () =>
+        {
+            tracked.SubscriberCount--;
+            if (tracked.SubscriberCount <= 0)
+            {
+                tracked.Subject.Dispose();
+                _querySubjects.Remove(query);
+            }
+        });
+    }
+
+    /// <summary>
+    /// Wraps an observable so that <paramref name="onLastDispose"/> is called
+    /// when the last subscriber disposes.
+    /// </summary>
+    private static Observable<EntityChange> WrapWithCleanup(
+        Observable<EntityChange> source, Action onLastDispose)
+    {
+        return Observable.Create<EntityChange>(observer =>
+        {
+            var subscription = source.Subscribe(observer);
+            return Disposable.Create(() =>
+            {
+                subscription.Dispose();
+                onLastDispose();
+            });
+        });
     }
 
     /// <summary>
@@ -99,14 +147,14 @@ internal class ChangeBuffer
     public void Dispose()
     {
         _entitySubject.Dispose();
-        foreach (var subject in _componentSubjects.Values)
+        foreach (var tracked in _componentSubjects.Values)
         {
-            subject.Dispose();
+            tracked.Subject.Dispose();
         }
         _componentSubjects.Clear();
-        foreach (var subject in _querySubjects.Values)
+        foreach (var tracked in _querySubjects.Values)
         {
-            subject.Dispose();
+            tracked.Subject.Dispose();
         }
         _querySubjects.Clear();
     }

src/OECS/ChangeSet.cs

@@ -4,13 +4,14 @@ namespace OECS;
 /// Accumulates <see cref="EntityChange"/> entries during a system run.
 ///
 /// Deduplication: if the same (entity, kind, componentType) change is marked
-/// multiple times, only one entry is kept. Entity-level changes (Added/Removed)
+/// multiple times, only one entry is kept. However, if a structurally opposed
-/// take precedence over component-level changes for the same entity.
+/// change arrives (Added vs Removed) for the same (entity, componentType),
+/// the old entry is removed so observers see the full state transition.
 /// </summary>
 internal class ChangeSet
 {
     private readonly List<EntityChange> _changes = new();
-    private readonly HashSet<(Entity Entity, ChangeKind Kind, Type? ComponentType)> _dedup = new();
+    private readonly Dictionary<(Entity Entity, ChangeKind Kind, Type? ComponentType), int> _dedup = new();
 
     /// <summary>
     /// All accumulated changes in insertion order.
@@ -75,9 +76,31 @@ internal class ChangeSet
     private void TryAdd(EntityChange change)
     {
         var key = (change.Entity, change.Kind, change.ComponentType);
-        if (_dedup.Add(key))
+        if (_dedup.ContainsKey(key))
         {
-            _changes.Add(change);
+            // Same (entity, kind, componentType) already recorded — deduplicate.
+            return;
         }
+
+        // When a component is re-added after being removed within the same
+        // batch, remove the old ComponentRemoved entry so observers see the
+        // full Add → Remove → Add sequence.
+        if (change.Kind == ChangeKind.ComponentAdded && change.ComponentType != null)
+        {
+            var removedKey = (change.Entity, ChangeKind.ComponentRemoved, change.ComponentType);
+            if (_dedup.Remove(removedKey, out int oldIndex))
+            {
+                _changes.RemoveAt(oldIndex);
+                // Adjust indices for all entries that shifted down.
+                foreach (var k in _dedup.Keys.ToList())
+                {
+                    if (_dedup[k] > oldIndex)
+                        _dedup[k]--;
+                }
+            }
+        }
+
+        _dedup[key] = _changes.Count;
+        _changes.Add(change);
     }
 }

src/OECS/QueryExecutor.cs

@@ -129,19 +129,95 @@ internal static class QueryExecutor
         var set3 = store.GetSet(typeof(T3)) as SparseSet<T3>;
         if (set3 == null) return;
 
-        var dense = set1.Dense;
+        // Pick the smallest set as the driver.
-        var denseEntities = set1.DenseEntities;
+        int c1 = set1.Count, c2 = set2.Count, c3 = set3.Count;
-        var count = set1.Count;
+
+        if (c2 <= c1 && c2 <= c3)
+        {
+            var dense = set2.Dense;
+            var denseEntities = set2.DenseEntities;
+            var count = set2.Count;
+            for (int i = 0; i < count; i++)
+            {
+                var entity = denseEntities[i];
+                if (entity.Id == SingletonId) continue;
+                if (!set1.Contains(entity)) continue;
+                if (!set3.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref set1.Get(entity), ref dense[i], ref set3.Get(entity));
+            }
+        }
+        else if (c3 <= c1 && c3 <= c2)
+        {
+            var dense = set3.Dense;
+            var denseEntities = set3.DenseEntities;
+            var count = set3.Count;
+            for (int i = 0; i < count; i++)
+            {
+                var entity = denseEntities[i];
+                if (entity.Id == SingletonId) continue;
+                if (!set1.Contains(entity)) continue;
+                if (!set2.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref set1.Get(entity), ref set2.Get(entity), ref dense[i]);
+            }
+        }
+        else
+        {
+            var dense = set1.Dense;
+            var denseEntities = set1.DenseEntities;
+            var count = set1.Count;
+            for (int i = 0; i < count; i++)
+            {
+                var entity = denseEntities[i];
+                if (entity.Id == SingletonId) continue;
+                if (!set2.Contains(entity)) continue;
+                if (!set3.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref dense[i], ref set2.Get(entity), ref set3.Get(entity));
+            }
+        }
+    }
+
+    private static void IterateThree<TDriver, TA, TB>(
+        SparseSet<TDriver> driveSet,
+        SparseSet<TA> setA,
+        SparseSet<TB> setB,
+        ComponentStore store,
+        IReadOnlySet<Type> withoutTypes,
+        ForEachAction<TA, TDriver, TB> actionWhenDriverIsT2,
+        ForEachAction<TDriver, TA, TB> actionWhenDriverIsT1,
+        ForEachAction<TA, TB, TDriver> actionWhenDriverIsT3)
+        where TDriver : struct where TA : struct where TB : struct
+    {
+        var dense = driveSet.Dense;
+        var denseEntities = driveSet.DenseEntities;
+        var count = driveSet.Count;
 
         for (int i = 0; i < count; i++)
         {
             var entity = denseEntities[i];
             if (entity.Id == SingletonId) continue;
-            if (!set2.Contains(entity)) continue;
+            if (!setA.Contains(entity)) continue;
-            if (!set3.Contains(entity)) continue;
+            if (!setB.Contains(entity)) continue;
-            if (!PassesWithoutFilter(store, entity, query.Without))
+            if (!PassesWithoutFilter(store, entity, withoutTypes))
                 continue;
-            action(entity, ref dense[i], ref set2.Get(entity), ref set3.Get(entity));
+
+            if (actionWhenDriverIsT2 != null!)
+            {
+                // Driver is T2, so order is: T1=setA, T2=driveSet, T3=setB
+                actionWhenDriverIsT2(entity, ref setA.Get(entity), ref dense[i], ref setB.Get(entity));
+            }
+            else if (actionWhenDriverIsT3 != null!)
+            {
+                // Driver is T3, so order is: T1=setA, T2=setB, T3=driveSet
+                actionWhenDriverIsT3(entity, ref setA.Get(entity), ref setB.Get(entity), ref dense[i]);
+            }
+            else
+            {
+                // Driver is T1, so order is: T1=driveSet, T2=setA, T3=setB
+                actionWhenDriverIsT1(entity, ref dense[i], ref setA.Get(entity), ref setB.Get(entity));
+            }
         }
     }
 
@@ -160,20 +236,73 @@ internal static class QueryExecutor
         var set4 = store.GetSet(typeof(T4)) as SparseSet<T4>;
         if (set4 == null) return;
 
-        var dense = set1.Dense;
+        // Pick the smallest set as the driver.
-        var denseEntities = set1.DenseEntities;
+        int c1 = set1.Count, c2 = set2.Count, c3 = set3.Count, c4 = set4.Count;
-        var count = set1.Count;
+        int min = Math.Min(Math.Min(c1, c2), Math.Min(c3, c4));
 
-        for (int i = 0; i < count; i++)
+        if (min == c2)
         {
-            var entity = denseEntities[i];
+            var dense = set2.Dense;
-            if (entity.Id == SingletonId) continue;
+            var denseEntities = set2.DenseEntities;
-            if (!set2.Contains(entity)) continue;
+            var count = set2.Count;
-            if (!set3.Contains(entity)) continue;
+            for (int i = 0; i < count; i++)
-            if (!set4.Contains(entity)) continue;
+            {
-            if (!PassesWithoutFilter(store, entity, query.Without))
+                var entity = denseEntities[i];
-                continue;
+                if (entity.Id == SingletonId) continue;
-            action(entity, ref dense[i], ref set2.Get(entity), ref set3.Get(entity), ref set4.Get(entity));
+                if (!set1.Contains(entity)) continue;
+                if (!set3.Contains(entity)) continue;
+                if (!set4.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref set1.Get(entity), ref dense[i], ref set3.Get(entity), ref set4.Get(entity));
+            }
+        }
+        else if (min == c3)
+        {
+            var dense = set3.Dense;
+            var denseEntities = set3.DenseEntities;
+            var count = set3.Count;
+            for (int i = 0; i < count; i++)
+            {
+                var entity = denseEntities[i];
+                if (entity.Id == SingletonId) continue;
+                if (!set1.Contains(entity)) continue;
+                if (!set2.Contains(entity)) continue;
+                if (!set4.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref set1.Get(entity), ref set2.Get(entity), ref dense[i], ref set4.Get(entity));
+            }
+        }
+        else if (min == c4)
+        {
+            var dense = set4.Dense;
+            var denseEntities = set4.DenseEntities;
+            var count = set4.Count;
+            for (int i = 0; i < count; i++)
+            {
+                var entity = denseEntities[i];
+                if (entity.Id == SingletonId) continue;
+                if (!set1.Contains(entity)) continue;
+                if (!set2.Contains(entity)) continue;
+                if (!set3.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref set1.Get(entity), ref set2.Get(entity), ref set3.Get(entity), ref dense[i]);
+            }
+        }
+        else
+        {
+            var dense = set1.Dense;
+            var denseEntities = set1.DenseEntities;
+            var count = set1.Count;
+            for (int i = 0; i < count; i++)
+            {
+                var entity = denseEntities[i];
+                if (entity.Id == SingletonId) continue;
+                if (!set2.Contains(entity)) continue;
+                if (!set3.Contains(entity)) continue;
+                if (!set4.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref dense[i], ref set2.Get(entity), ref set3.Get(entity), ref set4.Get(entity));
+            }
         }
     }
 
@@ -194,21 +323,94 @@ internal static class QueryExecutor
         var set5 = store.GetSet(typeof(T5)) as SparseSet<T5>;
         if (set5 == null) return;
 
-        var dense = set1.Dense;
+        // Pick the smallest set as the driver.
-        var denseEntities = set1.DenseEntities;
+        int c1 = set1.Count, c2 = set2.Count, c3 = set3.Count, c4 = set4.Count, c5 = set5.Count;
-        var count = set1.Count;
+        int min = Math.Min(Math.Min(Math.Min(c1, c2), Math.Min(c3, c4)), c5);
 
-        for (int i = 0; i < count; i++)
+        if (min == c2)
         {
-            var entity = denseEntities[i];
+            var dense = set2.Dense;
-            if (entity.Id == SingletonId) continue;
+            var denseEntities = set2.DenseEntities;
-            if (!set2.Contains(entity)) continue;
+            var count = set2.Count;
-            if (!set3.Contains(entity)) continue;
+            for (int i = 0; i < count; i++)
-            if (!set4.Contains(entity)) continue;
+            {
-            if (!set5.Contains(entity)) continue;
+                var entity = denseEntities[i];
-            if (!PassesWithoutFilter(store, entity, query.Without))
+                if (entity.Id == SingletonId) continue;
-                continue;
+                if (!set1.Contains(entity)) continue;
-            action(entity, ref dense[i], ref set2.Get(entity), ref set3.Get(entity), ref set4.Get(entity), ref set5.Get(entity));
+                if (!set3.Contains(entity)) continue;
+                if (!set4.Contains(entity)) continue;
+                if (!set5.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref set1.Get(entity), ref dense[i], ref set3.Get(entity), ref set4.Get(entity), ref set5.Get(entity));
+            }
+        }
+        else if (min == c3)
+        {
+            var dense = set3.Dense;
+            var denseEntities = set3.DenseEntities;
+            var count = set3.Count;
+            for (int i = 0; i < count; i++)
+            {
+                var entity = denseEntities[i];
+                if (entity.Id == SingletonId) continue;
+                if (!set1.Contains(entity)) continue;
+                if (!set2.Contains(entity)) continue;
+                if (!set4.Contains(entity)) continue;
+                if (!set5.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref set1.Get(entity), ref set2.Get(entity), ref dense[i], ref set4.Get(entity), ref set5.Get(entity));
+            }
+        }
+        else if (min == c4)
+        {
+            var dense = set4.Dense;
+            var denseEntities = set4.DenseEntities;
+            var count = set4.Count;
+            for (int i = 0; i < count; i++)
+            {
+                var entity = denseEntities[i];
+                if (entity.Id == SingletonId) continue;
+                if (!set1.Contains(entity)) continue;
+                if (!set2.Contains(entity)) continue;
+                if (!set3.Contains(entity)) continue;
+                if (!set5.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref set1.Get(entity), ref set2.Get(entity), ref set3.Get(entity), ref dense[i], ref set5.Get(entity));
+            }
+        }
+        else if (min == c5)
+        {
+            var dense = set5.Dense;
+            var denseEntities = set5.DenseEntities;
+            var count = set5.Count;
+            for (int i = 0; i < count; i++)
+            {
+                var entity = denseEntities[i];
+                if (entity.Id == SingletonId) continue;
+                if (!set1.Contains(entity)) continue;
+                if (!set2.Contains(entity)) continue;
+                if (!set3.Contains(entity)) continue;
+                if (!set4.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref set1.Get(entity), ref set2.Get(entity), ref set3.Get(entity), ref set4.Get(entity), ref dense[i]);
+            }
+        }
+        else
+        {
+            var dense = set1.Dense;
+            var denseEntities = set1.DenseEntities;
+            var count = set1.Count;
+            for (int i = 0; i < count; i++)
+            {
+                var entity = denseEntities[i];
+                if (entity.Id == SingletonId) continue;
+                if (!set2.Contains(entity)) continue;
+                if (!set3.Contains(entity)) continue;
+                if (!set4.Contains(entity)) continue;
+                if (!set5.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref dense[i], ref set2.Get(entity), ref set3.Get(entity), ref set4.Get(entity), ref set5.Get(entity));
+            }
         }
     }
 
@@ -231,22 +433,117 @@ internal static class QueryExecutor
         var set6 = store.GetSet(typeof(T6)) as SparseSet<T6>;
         if (set6 == null) return;
 
-        var dense = set1.Dense;
+        // Pick the smallest set as the driver.
-        var denseEntities = set1.DenseEntities;
+        int c1 = set1.Count, c2 = set2.Count, c3 = set3.Count, c4 = set4.Count, c5 = set5.Count, c6 = set6.Count;
-        var count = set1.Count;
+        int min = Math.Min(Math.Min(Math.Min(c1, c2), Math.Min(c3, c4)), Math.Min(c5, c6));
 
-        for (int i = 0; i < count; i++)
+        if (min == c2)
         {
-            var entity = denseEntities[i];
+            var dense = set2.Dense;
-            if (entity.Id == SingletonId) continue;
+            var denseEntities = set2.DenseEntities;
-            if (!set2.Contains(entity)) continue;
+            var count = set2.Count;
-            if (!set3.Contains(entity)) continue;
+            for (int i = 0; i < count; i++)
-            if (!set4.Contains(entity)) continue;
+            {
-            if (!set5.Contains(entity)) continue;
+                var entity = denseEntities[i];
-            if (!set6.Contains(entity)) continue;
+                if (entity.Id == SingletonId) continue;
-            if (!PassesWithoutFilter(store, entity, query.Without))
+                if (!set1.Contains(entity)) continue;
-                continue;
+                if (!set3.Contains(entity)) continue;
-            action(entity, ref dense[i], ref set2.Get(entity), ref set3.Get(entity), ref set4.Get(entity), ref set5.Get(entity), ref set6.Get(entity));
+                if (!set4.Contains(entity)) continue;
+                if (!set5.Contains(entity)) continue;
+                if (!set6.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref set1.Get(entity), ref dense[i], ref set3.Get(entity), ref set4.Get(entity), ref set5.Get(entity), ref set6.Get(entity));
+            }
+        }
+        else if (min == c3)
+        {
+            var dense = set3.Dense;
+            var denseEntities = set3.DenseEntities;
+            var count = set3.Count;
+            for (int i = 0; i < count; i++)
+            {
+                var entity = denseEntities[i];
+                if (entity.Id == SingletonId) continue;
+                if (!set1.Contains(entity)) continue;
+                if (!set2.Contains(entity)) continue;
+                if (!set4.Contains(entity)) continue;
+                if (!set5.Contains(entity)) continue;
+                if (!set6.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref set1.Get(entity), ref set2.Get(entity), ref dense[i], ref set4.Get(entity), ref set5.Get(entity), ref set6.Get(entity));
+            }
+        }
+        else if (min == c4)
+        {
+            var dense = set4.Dense;
+            var denseEntities = set4.DenseEntities;
+            var count = set4.Count;
+            for (int i = 0; i < count; i++)
+            {
+                var entity = denseEntities[i];
+                if (entity.Id == SingletonId) continue;
+                if (!set1.Contains(entity)) continue;
+                if (!set2.Contains(entity)) continue;
+                if (!set3.Contains(entity)) continue;
+                if (!set5.Contains(entity)) continue;
+                if (!set6.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref set1.Get(entity), ref set2.Get(entity), ref set3.Get(entity), ref dense[i], ref set5.Get(entity), ref set6.Get(entity));
+            }
+        }
+        else if (min == c5)
+        {
+            var dense = set5.Dense;
+            var denseEntities = set5.DenseEntities;
+            var count = set5.Count;
+            for (int i = 0; i < count; i++)
+            {
+                var entity = denseEntities[i];
+                if (entity.Id == SingletonId) continue;
+                if (!set1.Contains(entity)) continue;
+                if (!set2.Contains(entity)) continue;
+                if (!set3.Contains(entity)) continue;
+                if (!set4.Contains(entity)) continue;
+                if (!set6.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref set1.Get(entity), ref set2.Get(entity), ref set3.Get(entity), ref set4.Get(entity), ref dense[i], ref set6.Get(entity));
+            }
+        }
+        else if (min == c6)
+        {
+            var dense = set6.Dense;
+            var denseEntities = set6.DenseEntities;
+            var count = set6.Count;
+            for (int i = 0; i < count; i++)
+            {
+                var entity = denseEntities[i];
+                if (entity.Id == SingletonId) continue;
+                if (!set1.Contains(entity)) continue;
+                if (!set2.Contains(entity)) continue;
+                if (!set3.Contains(entity)) continue;
+                if (!set4.Contains(entity)) continue;
+                if (!set5.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref set1.Get(entity), ref set2.Get(entity), ref set3.Get(entity), ref set4.Get(entity), ref set5.Get(entity), ref dense[i]);
+            }
+        }
+        else
+        {
+            var dense = set1.Dense;
+            var denseEntities = set1.DenseEntities;
+            var count = set1.Count;
+            for (int i = 0; i < count; i++)
+            {
+                var entity = denseEntities[i];
+                if (entity.Id == SingletonId) continue;
+                if (!set2.Contains(entity)) continue;
+                if (!set3.Contains(entity)) continue;
+                if (!set4.Contains(entity)) continue;
+                if (!set5.Contains(entity)) continue;
+                if (!set6.Contains(entity)) continue;
+                if (!PassesWithoutFilter(store, entity, query.Without)) continue;
+                action(entity, ref dense[i], ref set2.Get(entity), ref set3.Get(entity), ref set4.Get(entity), ref set5.Get(entity), ref set6.Get(entity));
+            }
         }
     }
 }

src/OECS/World.cs

@@ -188,6 +188,17 @@ public class World : IDisposable
         // If replacing an existing relationship, remove the old index entry first.
         if (component is IRelationship newRel)
         {
+            // 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;