Investigation: Nullable owned entity data preservation when moving between parents

src/EFCore/ChangeTracking/Internal/ChangeDetector.cs

@@ -72,6 +72,25 @@ private static void ThrowIfKeyChanged(IInternalEntry entry, IProperty property)
         if (property.IsKey()
             && property.GetAfterSaveBehavior() == PropertySaveBehavior.Throw)
         {
+            // Allow key changes for owned entities when changing parent relationship in collections
+            // This is necessary when moving owned entities between different parent entity collections
+            if (entry is InternalEntityEntry entityEntry)
+            {
+                // Check if this property is part of an ownership foreign key
+                var entityType = entityEntry.EntityType;
+                var ownershipForeignKey = entityType.FindOwnership();
+                if (ownershipForeignKey != null && ownershipForeignKey.Properties.Contains(property))
+                {
+                    // Only allow the foreign key property change for owned entities in collection navigations
+                    // One-to-one owned relationships should still throw as they have different semantics
+                    var principalToDependent = ownershipForeignKey.PrincipalToDependent;
+                    if (principalToDependent != null && principalToDependent.IsCollection)
+                    {
+                        return;
+                    }
+                }
+            }
+
             throw new InvalidOperationException(CoreStrings.KeyReadOnly(property.Name, entry.StructuralType.DisplayName()));
         }
     }

src/EFCore/ChangeTracking/Internal/NavigationFixer.cs

@@ -1356,6 +1356,28 @@ private static void UndeleteDependent(
         if (dependentEntry.EntityState == EntityState.Deleted
             && principalEntry.EntityState is EntityState.Unchanged or EntityState.Modified)
         {
+            // For owned entities in collections, if the foreign key properties are part of the primary key,
+            // changing the parent means the primary key changes, so we need to treat this as
+            // a new entity (Added) rather than a modified entity
+            var entityType = dependentEntry.EntityType;
+            var ownership = entityType.FindOwnership();
+            if (ownership != null)
+            {
+                // Only apply this logic to owned entities in collection navigations
+                var principalToDependent = ownership.PrincipalToDependent;
+                if (principalToDependent != null && principalToDependent.IsCollection)
+                {
+                    // Check if any ownership foreign key properties are part of the primary key
+                    var keyProperties = entityType.FindPrimaryKey()?.Properties ?? Array.Empty<IProperty>();
+                    var ownershipFKProperties = ownership.Properties;
+                    if (keyProperties.Any(p => ownershipFKProperties.Contains(p)))
+                    {
+                        dependentEntry.SetEntityState(EntityState.Added);
+                        return;
+                    }
+                }
+            }
+
             dependentEntry.SetEntityState(EntityState.Modified);
         }
     }