cascade dropdownlist asp.net

Question

我在DB中有兩個表。

我想創建後that.this城市和相應的區域下拉列表是在我的整個life.i've我的第一個梯級drodown菜單試圖按照一些examples.in的取得動作我：

 ViewBag.cities = db.cities.ToList(); 
     ViewBag.areas = db.areas.ToList(); 

視圖

我：

 <div class="editor-field"> 
     @Html.DropDownListFor(model => model.city,new SelectList(ViewBag.cities as 
     System.Collections.IEnumerable, "city_id", "name"), 
     "Select City", new { id = "ddlCars" }) 
     @Html.ValidationMessageFor(model => model.city) 
     </div> 

    <div class="editor-label"> 
     @Html.LabelFor(model => model.area) 
    </div> 
    <div class="editor-field"> 
     @Html.DropDownListFor(model => model.area, new 
     SelectList(Enumerable.Empty<SelectListItem>(), "area_id", "name"), 
     "Select Area", new { id = "ddlModels" }) 
     @Html.ValidationMessageFor(model => model.area) 
    </div> 

我只是複製了一個網站，是

 $(document).ready(function() { 
     $("#ddlCars").change(function() { 
     var idModel = $(this).val(); 
     $.getJSON("/Post/LoadAreasByCity", { id: idModel }, 
     function (carData) { 
     var select = $("#ddlModels"); 
     select.empty(); 
     select.append($('<option/>', { 
     value: 0, 
     text: "Select Area" 
     })); 
     $.each(carData, function (index, itemData) { 

     select.append($('<option/>', { 
     value: itemData.Value, 
     text: itemData.Text 
     })); 
     }); 
     }); 
     }); 


     $("#ddlModels").change(function() { 
     var idColour = $(this).val(); 
     $.getJSON("/Home/LoadColoursByModel", { id: idColour }, 
     function (modelData) { 
     var select = $("#ddlColours"); 
     select.empty(); 
     select.append($('<option/>', { 
     value: 0, 
     text: "Select a Colour" 
     })); 
    $.each(modelData, function (index, itemData) { 

     select.append($('<option/>', { 
     value: itemData.Value, 
     text: itemData.Text 
     })); 
     }); 
     }); 
     }); 
     }); 

JS文件

在我的崗位/ LoadAreasByCity我的方法是：

  public JsonResult LoadAreasByCity(string id) 
      { 
      PostManager pm = new PostManager(); 
      if (id=="") { id = "1"; } 
      int c_id =(int) Convert.ToInt32(id); 
      var areaList = pm.GetModels(c_id); 

      var modelData = areaList.Select(m => new SelectListItem() 
       { 
       Text = m.name, 
       Value = m.area_id.ToString(), 

      }); 
      return Json(modelData, JsonRequestBehavior.AllowGet); 
      } 

它傳播城市和地區是正確的觀點page.but提交它給人的數據errorin此行

  @Html.DropDownListFor(model => model.city,new SelectList(ViewBag.cities as 
     System.Collections.IEnumerable, "city_id", "name"),"Select City", new { id = 
     "ddlCars" }) 

後它說值不能爲空。 參數名稱：項目

最後在我的崗位行動

   int c_id = Convert.ToInt32(p.city); 
       int a_id = Convert.ToInt32(p.area); 
       area a = db.areas.Single(x=>x.area_id == a_id); 
       city c = db.cities.Single(s => s.city_id == c_id); 
       post.city = c.name; 
       post.area = a.name; 
       ....Save to DB 

什麼是在這個問題.....在此先感謝

Answer 1

我懷疑，在您的文章，你的行動再次顯示了但同樣的觀點忘了填充ViewBag.cities和ViewBag.areas項目，如你在GET行動做：

[HttpPost] 
Public ActionResult Process() 
{ 
    ... Save to DB 

    ViewBag.cities = db.cities.ToList(); 
    ViewBag.areas = db.areas.ToList(); 
    return View(); 
}