如何扩展LINQ分部类

我需要扩展生成的亚音速(使用LINQ模板) 我在同一个项目中创建了具有相同名称和命名空间的类 当我运行这样的查询时

IMBDB db=new IMBDB();
var r = (from query in db.Articles
        join cat in db.ArticleCategories on query.CategoryID equals cat.ID
        where query.ID == articleId select new Article()
        {
            CategoryName = cat.Description,
            ID = query.ID
        });            
        return r.SingleOrDefault();
//The CategoryName is created in the extended partial class is always null
//while the generated sql is as expected

SELECT [t0].[ID], [t1].[Description] FROM [dbo].[Articles] AS t0
INNER JOIN [dbo].[ArticleCategories] AS t1
ON ([t0].[CategoryID] = [t1].[ID])
WHERE ([t0].[ID] = 40)

有什么办法可以纠正这个吗?

谢谢