Required dependent of optional dependent is not materialized/updated properly

[bachratyg]

An aggregate root has a 1:0..1 optional dependent with some required properties, using table splitting. This dependent entity has a 1:1 required dependent where all properties are nullable (that's why this nav is required). After the optional dependent has saved values, changes on the required dependent entity are not saved.

Steps to reproduce

Build & run.
Observe the last issued SQL command and "State before save:" line on the console output.
If the required dependent is replaced: "State before save: Added" and an update command.
If the required dependent is reused: "State before save: Detached" and no update command.
(note that both City and Street columns are updated in the former case despite the Street not changing)

using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Logging;

// Prep the db & seed
using( var db = new AppDb() )
{
	db.Database.EnsureDeleted();
	db.Database.EnsureCreated();
	db.Parents.Add(new() { Id = "x", Child = new() { Name = "asd" } });
	db.SaveChanges();
}

// reproduce the issue
using( var db = new AppDb() )
{
	var parent = db.Parents.Single();
	// This will update
	//parent.Child!.Address = new() { City = "asd" };
	// This will not
	parent.Child!.Address.City = "asd";
	db.ChangeTracker.DetectChanges();
	Console.WriteLine($"State before save: {db.Entry(parent.Child.Address).State}");
	db.SaveChanges();
}

// Model
class AppDb : DbContext
{
	protected override void OnConfiguring(DbContextOptionsBuilder builder)
	{
		builder
			.UseSqlServer("Data Source=localhost;Initial Catalog=Test;Integrated Security=true")
			.LogTo(Console.WriteLine, LogLevel.Information)
			.EnableSensitiveDataLogging()
			;
	}
	protected override void OnModelCreating(ModelBuilder builder)
	{
		builder.Entity<Parent>().OwnsOne(e => e.Child).OwnsOne(e => e.Address);
	}
	public DbSet<Parent> Parents => this.Set<Parent>();
}

class Parent
{
	public required string Id { get; set; }
	public Child? Child { get; set; }
}
class Child
{
	public required string Name { get; set; }
	public Address Address { get; set; } = new();
}
class Address
{
	// All optional, hence the navigation to this entity is required
	public string? City { get; set; }
	public string? Street { get; set; }
}

First I thought this has to do with owned entities and thus part of #24581 and #1985 however it repros with plain old navigations. Replace OnModelCreating with the following (basically maps to the same table structure), steps and outcome is the same

	protected override void OnModelCreating(ModelBuilder builder)
	{
		builder.Entity<Parent>().ToTable("Parents");
		builder.Entity<Parent>().HasOne(e => e.Child).WithOne().HasForeignKey<Child>();
		builder.Entity<Parent>().Navigation(e => e.Child).AutoInclude();
		builder.Entity<Child>().Property<string>("Id");
		builder.Entity<Child>().HasKey("Id");
		builder.Entity<Child>().ToTable("Parents");
		builder.Entity<Child>().HasOne(e => e.Address).WithOne().HasForeignKey<Address>();
		builder.Entity<Child>().Navigation(e => e.Address).IsRequired().AutoInclude();
		builder.Entity<Address>().Property<string>("Id");
		builder.Entity<Address>().HasKey("Id");
		builder.Entity<Address>().ToTable("Parents");
	}

Some additional observations:

• the navigation is required but warns OptionalDependentWithAllNullPropertiesWarning[20704] and neither the Address nor the City and Street setters are hit on materialization
• the Address is Detached on materialization, it should be Unchanged
• even though Detached the change tracker still maintains some knowledge of the Address or else the replace/reuse behavior could not be possibly different. It's just an unknown object.

Include provider and version information

EF Core version: 8.0.8
Database provider: Microsoft.EntityFrameworkCore.SqlServer (probably others)
Target framework: .NET 8.0