ASP.NET MVC3 在controller中怎么为新建的记录新建关联记录

public class CompanyInfo    {        [Key]        [Column("id")]        public int id { get; set; }        public virtual List<CompanyInfoActHistory> CompanyInfoActHistorys { get; set; } //客户流水记录表                //省略部分字段……    }

    public class CompanyInfoActHistory    {        [Key]        public int id { get; set; }        [Column("CompanyId")]        public int CompanyInfoId { get; set; }        [Column("Actdate")]        public DateTime Actdate { get; set; }        [Column("UserId")]        public int OperatorId { get; set; }        [Column("ip")]        public string IP { get; set; }        [Column("Content")]        public string Content { get; set; }        public virtual CompanyInfo companyInfo { get; set; }        public virtual Operator Operator { get; set; }    }

[HttpPost]        public ActionResult Create(CompanyInfo companyInfo)        {            if (ModelState.IsValid)            {                CompanyInfoActHistory actHistory = new CompanyInfoActHistory()                {                    Actdate = System.DateTime.Now,                    OperatorId = Convert.ToInt32(System.Web.HttpContext.Current.Request.Cookies["POM"]["Adminid"]),                    IP = System.Web.HttpContext.Current.Request.UserHostAddress == null ? "" : System.Web.HttpContext.Current.Request.UserHostAddress,                    Content = this.SetContent()                };                companyInfo.CompanyInfoActHistorys.Add(actHistory);                db.CompanyInfo.Add(companyInfo);                db.SaveChanges();                return Content("成功新建记录", "text/html");            }            return Content("未成功新建记录", "text/html");        }