如何在.netframework 3.5 / Code First Model中创建外键关系？

Question

我的.net framework 4.0项目与外键关系有下面的数据合约。我想为.net framework 3.5项目重用相同的代码。但我错过了ForeignKey属性的错误。我如何重写.netframework 3.5的以下代码？请帮助。

using System;
using System.Collections.Generic;
Using System.ComponentModel.DataAnnotations;
Using System.ComponentModel.DataAnnotations.Schema;

namespace xyz {
    [DataContract]
    public class Category
    {       
        public string category_id { get; set; }  

        public string category_name { get; set; }    

        public virtual ICollection<SubCategory> SubCategorys { get; set; }
    }


    [DataContract]

    public class SubCategory 
    {
        public string sub_category_id { get; set; }
        public string sub_category_name { get; set; }
        public virtual ICollection<SubCategory2> SubCategory2s { get; set; }
        [ForeignKey("category_id")]
        public virtual Category Category { get; set; }

    }

    [DataContract]
    public class SubCategory2 
    {
    public string sub_category2_id { get; set; }
    public string sub_category2_name { get; set; }
    [ForeignKey("sub_category_id")]
    public virtual SubCategory SubCategory { get; set; }
    }
}