MVC+EF 序列化类型为“System.Data.Entity.DynamicProxies.__的对象时检测到循环引用

原代码：
 public ActionResult JSon({
            NorthwindEntities db = new NorthwindEntities();
            Employees per = db.Employees.Where(u => u.EmployeeID == 1).FirstOrDefault();
            return Json(per, JsonRequestBehavior.AllowGet);
}

1 public ActionResult JSon(){
2             NorthwindEntities db = new NorthwindEntities();
3             db.Configuration.ProxyCreationEnabled = false;
4             Employees per = db.Employees.Where(u => u.EmployeeID == 1).FirstOrDefault();
5             return Json(per, JsonRequestBehavior.AllowGet);
6 }