突然搞不懂貌似应该很简单的CollectionViewSource的Grouping问题了

目的：Northwind数据库的Product表按CategoryName(或CategoryId)分组显示。

数据库就这么简单了：


//NorthwindDomainService.metadata
internal sealed class ProductMetadata
{
...
[Include]
public Category Category { get; set; }
...

//NorthwindDomainService.cs
public IQueryable<Product> GetProducts()
{
return this.ObjectContext.Products.Include("Category") ;
}

这些都做了。

//HomeViewModel.cs

public class HomeViewModel

{

    public IEnumerable<Product> Products { get; set; }

    public HomeViewModel()

    {

        var context = new NorthwindDomainContext();

        Products = context.Products;

        if (!DesignerProperties.IsInDesignTool)

            context.Load(context.GetProductsQuery());

    }

}

Home.xaml
xmlns:vm="clr-namespace:GroupingTest002.ViewModels"
...
<navigation:Page.Resources>
<CollectionViewSource x:Key="ProductsCV" Source="{Binding Products}">

</CollectionViewSource>
</navigation:Page.Resources>
<navigation:Page.Style>
<StaticResource ResourceKey="PageStyle"/>
</navigation:Page.Style>
<navigation:Page.DataContext>
<vm:HomeViewModel />
</navigation:Page.DataContext>

...
<sdk:DataGrid Height="300" ItemsSource="{Binding Source={StaticResource ProductsCV}}" AutoGenerateColumns="False">
<sdk:DataGrid.Columns>
<sdk:DataGridTextColumn Header="Product Id" SortMemberPath="ProductID" Binding="{Binding ProductID}" />
<sdk:DataGridTextColumn Header="Product Name" SortMemberPath="ProductName" Binding="{Binding ProductName}" />
<sdk:DataGridTextColumn Header="Cateogry" SortMemberPath="CategoryId" Binding="{Binding Category.CategoryName}" />
</sdk:DataGrid.Columns>
</sdk:DataGrid>

目前为止显示没问题。

但是，想用CategoryId分组没效果, PropertyName设成Category, Category.CategoryName等等都不行。但用Product表的ReorderLevel, Discontinued等是可以分组的。
<CollectionViewSource x:Key="ProductsCV" Source="{Binding Products}">
<CollectionViewSource.GroupDescriptions>
<PropertyGroupDescription PropertyName="CategoryId" />
</CollectionViewSource.GroupDescriptions>
</CollectionViewSource>

难道不能用外键字段进行分组吗？