Fix error when moving child entities between parents with DeleteBehavior.NoAction

src/EFCore/ChangeTracking/Internal/InternalEntryBase.cs

@@ -1441,8 +1441,13 @@ public virtual void HandleNullForeignKey(
                 SetPropertyModified(property, isModified: true, isConceptualNull: true);
             }
 
+            // For NoAction relationships, defer conceptual null validation to allow for entity reparenting
+            var shouldDeferValidation = property.GetContainingForeignKeys()
+                .Any(fk => fk.DeleteBehavior == DeleteBehavior.NoAction);
+
             if (!isCascadeDelete
-                && StateManager.DeleteOrphansTiming == CascadeTiming.Immediate)
+                && StateManager.DeleteOrphansTiming == CascadeTiming.Immediate
+                && !shouldDeferValidation)
             {
                 HandleConceptualNulls(
                     StateManager.SensitiveLoggingEnabled,

test/EFCore.Tests/ChangeTracking/Internal/ChildMoveTest.cs

@@ -0,0 +1,146 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Microsoft.EntityFrameworkCore.TestUtilities;
+
+namespace Microsoft.EntityFrameworkCore.ChangeTracking.Internal;
+
+public class ChildMoveTest
+{
+    [ConditionalFact]
+    public async Task Moving_child_between_parents_with_NoAction_should_not_throw()
+    {
+        using var context = new TestContext();
+        await context.Database.EnsureDeletedAsync();
+        await context.Database.EnsureCreatedAsync();
+
+        // Arrange - Create initial data
+        var fromParent = new Parent { Id = 1, Name = "From" };
+        var toParent = new Parent { Id = 2, Name = "To" };
+        var child = new Child { Id = 3, Name = "Child" };
+
+        fromParent.Children.Add(child);
+
+        context.Parents.Add(fromParent);
+        context.Parents.Add(toParent);
+        context.Children.Add(child);
+
+        await context.SaveChangesAsync();
+
+        // Act - Move child from one parent to another
+        fromParent.Children.Remove(child);
+        toParent.Children.Add(child);
+
+        // This should not throw an exception
+        await context.SaveChangesAsync();
+
+        // Assert - Verify child is now associated with toParent
+        var updatedChild = await context.Children.FindAsync(3);
+        var updatedToParent = await context.Parents.Include(p => p.Children).FirstAsync(p => p.Id == 2);
+        var updatedFromParent = await context.Parents.Include(p => p.Children).FirstAsync(p => p.Id == 1);
+
+        Assert.Contains(updatedChild, updatedToParent.Children);
+        Assert.DoesNotContain(updatedChild, updatedFromParent.Children);
+    }
+
+    [ConditionalFact]
+    public async Task Orphaning_child_with_NoAction_should_still_throw()
+    {
+        using var context = new TestContext();
+        await context.Database.EnsureDeletedAsync();
+        await context.Database.EnsureCreatedAsync();
+
+        // Arrange - Create initial data
+        var parent = new Parent { Id = 1, Name = "Parent" };
+        var child = new Child { Id = 2, Name = "Child" };
+
+        parent.Children.Add(child);
+
+        context.Parents.Add(parent);
+        context.Children.Add(child);
+
+        await context.SaveChangesAsync();
+
+        // Act - Remove child from parent without adding to another parent (orphaning)
+        parent.Children.Remove(child);
+
+        // This should still throw an exception for truly orphaned children
+        var exception = await Assert.ThrowsAsync<InvalidOperationException>(() => context.SaveChangesAsync());
+
+        Assert.Contains("association between entity types 'Parent' and 'Child' has been severed", exception.Message);
+    }
+
+    [ConditionalFact]
+    public async Task Moving_multiple_children_between_parents_with_NoAction_should_work()
+    {
+        using var context = new TestContext();
+        await context.Database.EnsureDeletedAsync();
+        await context.Database.EnsureCreatedAsync();
+
+        // Arrange - Create initial data with multiple children
+        var fromParent = new Parent { Id = 1, Name = "From" };
+        var toParent = new Parent { Id = 2, Name = "To" };
+        var child1 = new Child { Id = 3, Name = "Child1" };
+        var child2 = new Child { Id = 4, Name = "Child2" };
+
+        fromParent.Children.Add(child1);
+        fromParent.Children.Add(child2);
+
+        context.Parents.Add(fromParent);
+        context.Parents.Add(toParent);
+        context.Children.AddRange(child1, child2);
+
+        await context.SaveChangesAsync();
+
+        // Act - Move multiple children from one parent to another
+        fromParent.Children.Remove(child1);
+        fromParent.Children.Remove(child2);
+        toParent.Children.Add(child1);
+        toParent.Children.Add(child2);
+
+        // This should not throw an exception
+        await context.SaveChangesAsync();
+
+        // Assert - Verify both children are now associated with toParent
+        var updatedToParent = await context.Parents.Include(p => p.Children).FirstAsync(p => p.Id == 2);
+        var updatedFromParent = await context.Parents.Include(p => p.Children).FirstAsync(p => p.Id == 1);
+
+        Assert.Equal(2, updatedToParent.Children.Count);
+        Assert.Empty(updatedFromParent.Children);
+        Assert.Contains(updatedToParent.Children, c => c.Name == "Child1");
+        Assert.Contains(updatedToParent.Children, c => c.Name == "Child2");
+    }
+
+    private class TestContext : DbContext
+    {
+        public DbSet<Parent> Parents { get; set; }
+        public DbSet<Child> Children { get; set; }
+
+        protected internal override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
+            => optionsBuilder
+                .UseInternalServiceProvider(InMemoryFixture.DefaultServiceProvider)
+                .UseInMemoryDatabase("ChildMoveTest");
+
+        protected internal override void OnModelCreating(ModelBuilder modelBuilder)
+        {
+            modelBuilder.Entity<Parent>()
+                .HasMany(p => p.Children)
+                .WithOne()
+                .IsRequired()
+                .OnDelete(DeleteBehavior.NoAction);
+        }
+    }
+
+    private class Child
+    {
+        public int Id { get; set; }
+        public string Name { get; set; }
+    }
+
+    private class Parent
+    {
+        public int Id { get; set; }
+        public required string Name { get; set; }
+        public ICollection<Child> Children { get; set; } = new List<Child>();
+    }
+}