求教高手，Silverlight RIA Service 数据表关联自定义类及查询有关问题

求教高手，Silverlight RIA Service 数据表关联自定义类及查询问题？
数据库中有两个表，一个地点表Site(字段有SiteID，Longitude，Latitude,Height,SiteName)，一个调查表Survey(字段有SurveyID,SiteID,Time,Num),然后生成实体模型，建立DomainService服务类，我是在DomainService.metadata中自定义了一个非实体类
    [DataContract]
    public class Site_SurveyInfo    {   
       [DataMember]
        [Key]
        public int  siteID { get; set; }
         [DataMember]
        public decimal latitude      { get; set; }
        [DataMember]
        public decimal longitude     { get; set; }
        [DataMember]
        public float   height         {get;set;}
        [DataMember]
        public string siteName  { get; set; }
        [DataMember]
        public DateTime  time           { get; set; }
        [DataMember]
        public int   num { get; set; }
               
    }

然后在DomainService中自定义一个查询方法
         [Query]
        public IQueryable<Site_SurveyInfo> GetSite_SurveyInfo()
        {
            IQueryable<Site_SurveyInfo> query = from a in this.ObjectContext.Site    join b in this.ObjectContext.Survey on a.SiteID equals b.SiteID
                        select new Site_SurveyInfo                       
            {
                             siteID=a.SiteID,
                             longitude = a.Longitude,
                             latitude = a.Latitude,
                             height=a.Height,  
                             siteName = a.SiteName,