Weird data binding behavior in ASP.NET MVC3?

I have the following domain objects:

public class Foo  
{  
    public Id {get; set;}   
    **public BarId {get; set;}**   
    public virtual Bar {get; set;}  
    public string Name {get; set;}  
}  
public class Bar  
{
    public Id {get; set;}     
    public string 开发者_JAVA技巧Name {get; set;}  
}

The view model used by the view looks like this:

public class FooEditorModel  
{  
   public Foo {set; get;}
   public SelectList Bars { get; set; }
}  

The view looks like this:

@model FooEditorModel  
@Html.ValidationSummary( true )  
@Html.HiddenFor( model => model.Foo.Id )  
@Html.LabelFor( model => model.Foo.BarId, "Bar" )  
@Html.DropDownListFor( model => model.Foo.BarId, Model.Bars )  
@Html.ValidationMessageFor( model => model.Foo.BarId )

The controller looks like this:

[HttpPost]  
public virtual ActionResult Save( FooEditorModel model )
{  
    if ( ModelState.IsValid )
    {
        Foo foo = MakeAModel( model );
        FooRepository.Save( foo );
    }
}  

Foo MakeAModel( model )
{
   Foo foo = MakeAFoo();
   **foo.BarId = model.Foo.BarId;**
   return foo;
}

Quite standard code, a bit simplified.

The problem is that if I use a Bar in Foo, instead of BarId, the ModelState is not valid (because of Bar.Name).

I find it very poorly designed to be forced to have both a child Id and a reference to the child in the root object.

Is this the only way for a data binding to work in MS MVC 3?