wcf data services savechanges():“找不到该段的资源”

我的台词 ctx.SaveChanges(); 在我的控制器中 DutyController.cs 当我试图编辑我的职责对象时引发以下异常。

System.Data.Services.Client.DataServiceClientException:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<error xmlns="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata">
  <code></code>
  <message xml:lang="en-US">Resource not found for the segment 'Duty'.</message>
</error>

我的“编辑职责”视图包括一个下拉列表的场合。occasionid guid在提交时成功绑定到工作模型。

模特儿

[MetadataType(typeof(DutyMetadata))]
public partial class Duty
{
    private class DutyMetadata
    {
        ...

        [Required]
        [UIHint("OccasionId")]
        [Display(Name = "Occasion")]
        public object OccasionId { get; set; }
    }
}

视图\职责\编辑.aspx

<%: Html.EditorForModel() %>

视图\共享\显示模板\occasionid.ascx

<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<Guid>" %>
<%: Html.DropDownList(string.Empty, new SelectList(ViewData["occasions"] as IEnumerable<Web.VolunteerData.Occasion>, "Id", "Designation", Model), "Select Occasion")%>

控制器\dutycontroller.cs

//
// GET: /Duty/Edit/5

public ActionResult Edit(Guid id)
{
    var Model = (from Duty d in ctx.Duties where d.Id == id select d).Single();
    ViewData["occasions"] = from Occasion o in ctx.Occasions orderby o.Designation select o;
    return View(Model);
}

//
// POST: /Duty/Edit/5

[HttpPost]
public ActionResult Edit(Duty Model)
{
    ctx.AttachTo("Duty", Model);
    ctx.UpdateObject(Model);
    ctx.SaveChanges();
    return RedirectToAction("Index");
}

为什么savechanges()会导致“找不到段的资源…”异常?

有没有更好的方法让我做我想做的?