/// <summary>
        ///获得多个数据到St扩展属性udens类的
        /// </summary>
        /// <param name="classId"></param>
        /// <returns></returns>
        public List<Students> GetStuList(int classId)
        {
            //根据ClassId从StudentClass中取ClassName，并给Student类中的扩展属性ClassName
            return Db.Queryable<Students, StudentClass>((stu, stuClass) => new object[] { JoinType.Inner, stu.ClassId == stuClass.ClassId })
                .Where(stu => stu.ClassId == classId).Select((stu,stuClass)=>new Students() {StudentName=stu.StudentName.SelectAll(), ClassName=stuClass.ClassName }).ToList();
        }