Required Field not being validated by ModelState

I have a [Products] class, a [Product_Types] class and a [Products_Product_Types] class. I have a Razor page for [Products_Product_Types] and ModelState is only validating one of the two [Required] ID properties...specifically it's validating the ProductProductType.ProductId property and ignoring the ProductProductType.ProductTypeId property. In the code samples below, when I leave the ProductProductType.ProductTypeId blank I get this error...

> The value of 'ProductProductType.ProductTypeId' is unknown when
> attempting to save changes. This is because the property is also part
> of a foreign key for which the principal entity in the relationship is
> not known.

...when instead I expect to see the UnobtrusiveValidation that the ProductProductType.ProductTypeId dropdown is required.

I fully expect the issue is in my DatabaseContext.cs file, but I've reviewed all the documentation I can find and I'm not sure how to resolve this error.

Product.cs

public partial class Product
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int ProductId { get; set; }

    [Display(Name = "Product Name")]
    public string? ProductName { get; set; }
}

ProductType.cs

public partial class ProductType
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int ProductTypeId { get; set; }

    [Display(Name = "Product Type")]
    public string? ProductTypeName { get; set; }
}

ProductsProductType.cs

public partial class ProductsProductType
{
    [Required]
    public int ProductId { get; set; }

    [Required]
    [Display(Name = "Product Type")]
    public int ProductTypeId { get; set; }

    public virtual Product? Product { get; set; } = null!;
    //public virtual ICollection<Product>? Products { get; set; } = new List<Product>();

    public virtual ProductType? ProductType { get; set; } = null!;
    //public virtual ICollection<ProductType>? ProductTypes { get; set; } = new List<ProductType>();
}

DatabaseContext.cs

public virtual DbSet<ProductsProductType> ProductsProductTypes{ get; set; }

modelBuilder.Entity<ProductsProductType>(entity =>
{
    entity.HasKey(e => new { e.ProductId, e.ProductTypeId }).HasName("PK_Products_Product_Types");

    entity.ToTable("Products_Product_Types");

    entity.Property(e => e.ProductId).HasColumnName("ProductID");
    entity.Property(e => e.ProductTypeID).HasColumnName("ProductTypeID");

    entity.HasOne(f => f.Product).WithMany(e => e.ProductsProductTypes)
        .HasForeignKey(f => f.ProductId).IsRequired()
        .OnDelete(DeleteBehavior.ClientSetNull)
        .HasConstraintName("FK_Products_Product_Types_Products");

    entity.HasOne(f => f.ProductType).WithMany(e => e.ProductsProductTypes)
        .HasForeignKey(f => f.ProductTypeID).IsRequired()
        .OnDelete(DeleteBehavior.ClientSetNull)
        .HasConstraintName("FK_Product_Product_Types_Product_Types");
});

/ProductsProductTypes/Create.cshtml

<form method="post" asp-page-handler="ProductProductTypeCreate" data-ajax="true" data-ajax-method="post" data-ajax-success="refreshPartial('GetProductProductTypes', 'ProductProductTypes-container')" data-ajax-failure="ajaxValidationHandler">
            <div asp-validation-summary="ModelOnly" class="text-danger"></div>
            <input type="hidden" asp-for="ProductProductType.ProductId" value="@ViewBag.ProductId" />
            <div class="form-group">
                <label asp-for="ProductProductType.ProductTypeId" class="control-label"></label>
                <select asp-for="ProductProductType.ProductTypeId" class="form-control" asp-items="ViewBag.ProductTypes"><option disabled selected>- Select Product Type -</option></select>
                <span asp-validation-for="ProductProductType.ProductTypeId" class="text-danger"></span>
            </div>
            <div class="form-group">
                <input type="submit" value="Save" class="btn btn-primary" />
                <a class="btn btn-secondary" href="javascript:parent.actionPaneClose();">Cancel</a>
            </div>
        </form>

/ProductsProductTypes/Create.cshtml.cs

[BindProperty]
public ProductsProductType ProductsProductType { get; set; } = default!;

public async Task<IActionResult> OnPostProductProductTypeCreateAsync()
{
    if (!ModelState.IsValid || _context.ProductsProductTypes == null || ProductsProductType == null)
    {
        ViewData["ProductId"] = ProductsProductType.ProductId;
        ViewData["ProductTypes"] = new SelectList(_context.ProductTypes, nameof(ProductType.ProductTypeId), nameof(ProductType.ProductTypeName));
        return Page();
    }

    _context.ProductsProductTypes.Add(ProductsProductType);
    await _context.SaveChangesAsync();

    return Page();
}

You could check the document related with Non-nullable reference types and the [Required] attribute

> On the server, a required value is considered missing if the property
> is null. A non-nullable field is always valid, and the [Required]
> attribute's error message is never displayed.
>
> However, model binding for a non-nullable property may fail, resulting
> in an error message such as The value '' is invalid.

When you submit the form if you don't set any value in input field,you would see a key-value pair key-"",but if you don't select any option of the dropdown,you won't see the key-value pair

I tried to remove value attribute in your hidden input,disable client validation and submit the form:

<input type="hidden" asp-for="ProductsProductType.ProductId"  />

and get the modelstate err due to failed model binding(if you want to avoid it you may create a viewmodel with int? xxId):

The error

> The value of 'ProductProductType.ProductTypeId' is unknown when
> attempting to save changes. This is because the property is also part
> of a foreign key for which the principal entity in the relationship is
> not known.

ask you to send the ProductTypeId matches ProductType entity's PK in db

Since you've read all your options from db

ViewData["ProductTypes"] = new SelectList(_context.ProductTypes, nameof(ProductType.ProductTypeId), nameof(ProductType.ProductTypeName));

make sure you've selected one of the options and enabled client validation

like:

@section Scripts {
    @{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
}

in _ValidationScriptsPartial:

<script src="~/lib/jquery-validation/dist/jquery.validate.min.js"></script>
<script src="~/lib/jquery-validation-unobtrusive/jquery.validate.unobtrusive.min.js"></script>

if you still can't enable the client validation,please show us a minimal example of your current page and layout view