TryUpdateModelAsync rejects null naviagation properties?

TryUpdateModelAsync() seems to reject my model objects if the navigation property is null, even though the foreign key is properly set.

For example, it won't accept a Book if AuthorId is set, but Author is null:

public class Book
{
    public int Id { get; set; }
    public string Title { get; set; }
    public int AuthorId { get; set; }
    public Author Author { get; set; }
}

Here is a complete working example showing the problem of this behavior:

public class Author
{
    public int Id { get; set; }
    public string Name { get; set; }
    public List<Book> Books { get; set; }
}

Model:

public class EditModel : PageModel
{
    private readonly YourDbContext _context;
    private readonly ILogger<EditModel> _logger;

    [BindProperty] public Book Book { get; set; }
    public List<Author> Authors {get; set;}

    public EditModel(YourDbContext context, ILogger<EditModel> logger)
    {
        _context = context;
        _logger = logger;
    }

    public async Task<IActionResult> OnGetAsync(int? id)
    {
        if (id == null) { return NotFound(); }

        Book = await _context.Books.Include(b => b.Author).FirstOrDefaultAsync(m => m.Id == id);
        if (Book == null) { return NotFound(); }

        Authors = await _context.Authors.ToListAsync();
        return Page();
    }

    public async Task<IActionResult> OnPostAsync(int? id)
    {
        var bookToUpdate = await _context.Books.FindAsync(id);
        if (bookToUpdate == null) { return NotFound(); }

        if (await TryUpdateModelAsync<Book>(bookToUpdate, "book", b => b.Title, b => b.AuthorId))
        {
            await _context.SaveChangesAsync();
            return RedirectToPage("./Index");
        }
        else 
        {
            foreach (var modelStateKey in ModelState.Keys)
            {
                var modelStateVal = ModelState[modelStateKey];
                foreach (var error in modelStateVal.Errors)
                {
                    _logger.LogError($"Key: {modelStateKey}, Error: {error.ErrorMessage}");
                }
            }
        }
        Book = bookToUpdate;
        return RedirectToPage(new { id});
    }
}

Razor Page:

@page
@model EditModel

<form method="post">
    <input type="hidden" asp-for="Book.Id" />
    <input asp-for="Book.Title" />
    <select asp-for="Book.AuthorId" asp-items="@(new SelectList(Model.Authors,"Id","Name"))"></select>
    <input type="submit" value="Save" />
</form>

The modelstate will always have errors complaining:

Key: Book.Author, Error: The Author field is required.

This is obviously a simplified example, but in general there can be many large navigation properties, and so populating them all is not feasible.

In .Net 6 or higher versions, it would enable nullable reference types atuomaticlly in your csproj file:

<Nullable>enable</Nullable>

The property

public Author Author { get; set; }

would be considered as non-nullable

And as mentioned in the document:

> The validation system treats non-nullable parameters or bound
> properties as if they had a [Required(AllowEmptyStrings = true)]
> attribute.

If you want to keep it as non-nullable,the solution for MVC projects:

builder.Services.AddControllers(
    options => options.SuppressImplicitRequiredAttributeForNonNullableReferenceTypes = true);

In Razor Page projects,you could try:

builder.Services.Configure<MvcOptions>(x=>x.SuppressImplicitRequiredAttributeForNonNullableReferenceTypes=true);

I tried on myside and it works :