我有一个MVC3 C＃项目，我有一个FoodItem和FoodItemCategory模型。这两个模型如下所示：
public class FoodItem
{
public int ID { get; set; }
[Required]
public string Name { get; set; }
public string Description { get; set; }
public virtual ICollection<FoodItemCategory> Categories { get; set; }
public DateTime CreateDate { get; set; }
}
public class FoodItemCategory {
public int ID { get; set; }
public string Name { get; set; }
public string Description { get; set; }
public virtual ICollection<FoodItem> FoodItems { get; set; }
public DateTime CreateDate { get; set; }
}
我有一个最初从Scaffolder生成的_CreateOrEdit.cshtml视图，我将其修改为包含所有类别并检查食品所属的框。食品可以具有许多或所有类别。该视图如下所示：
@model StackOverFlowIssue.Models.FoodItem
<div class="editor-label">
@Html.LabelFor(model => model.Name)
</div>
<div class="editor-field">
@Html.EditorFor(model => model.Name)
@Html.ValidationMessageFor(model => model.Name)
</div>
<div class="editor-label">
@Html.LabelFor(model => model.Description)
</div>
<div class="editor-field">
@Html.EditorFor(model => model.Description)
@Html.ValidationMessageFor(model => model.Description)
</div>
<div class="editor-label">
@Html.LabelFor(model => model.Categories)
</div>
<div class="editor-field">
@foreach (var FoodItemCategory in (IEnumerable<StackOverFlowIssue.Models.FoodItemCategory>)ViewBag.Categories){
<input type="checkbox" name="FoodItemCategoryId" value="@FoodItemCategory.ID"
@foreach(var c in Model.Categories){
if(c.ID == FoodItemCategory.ID){
@String.Format("checked=\"checked\"")
}
}
/>
@FoodItemCategory.Name
<br />
}
</div>
@Html.Hidden("CreateDate", @DateTime.Now)
正如您所看到的，我有一个嵌套循环，为每个类别创建一个复选框，当它创建每个类别时，我循环并检查我的模型的Categories属性中的特定类别。如果存在，我设置复选框的checked属性。如果您选中一个框并单击保存，则在控制器上的HttpPost操作上，我将执行以下操作：
[HttpPost]
public ActionResult Edit(FoodItem foodItem)
{
if (ModelState.IsValid)
{
var cList = Request["CategoryId"].Split(',');
List<FoodItemCategory> categories = new List<FoodItemCategory>();
foreach (var c in cList) {
var ci = Convert.ToInt32(c);
FoodItemCategory category = context.FoodItemCategories.Single(x => x.ID == ci);
categories.Add(category);
}
context.Entry(foodItem).State = EntityState.Modified;
restaurant.Categories = categories;
context.SaveChanges();
return RedirectToAction("Index");
}
return View(foodItem);
}
我可以保存一次类别。如果我回到视图中，只需单击“保存”，我会收到以下错误：
无法将重复值插入唯一索引。 [表名=＆gt; FoodItemCategoryFoodItems，约束名称= PK_ FoodItemCategoryFoodItems _00000000000000A8] 描述：执行当前Web请求期间发生未处理的异常。请>查看堆栈跟踪，以获取有关错误及其在代码中的起源位置的更多信息。
异常详细信息：System.Data.SqlServerCe.SqlCeException：无法将重复值插入到＆gt;唯一索引中。 [表名= FoodItemCategoryFoodItems，约束名称=＆gt; PK_ FoodItemCategoryFoodItems _00000000000000A8]
来源错误：
第97行：context.Entry（foodItem）.State = EntityState.Modified; 第98行：foodItem.Categories =类别; 第99行：context.SaveChanges（）; 第100行：返回RedirectToAction（“Index”）; 第101行：}
不确定是否重要，但我使用的是SQLServer Compact Edition 4.我是否正确地采用了这种方式？对于这样的事情，正常的编码实践是什么？我知道同样的情况每天都会发生，因为在博客等许多情况下使用相同的关系模型
答案 0 :(得分：4)
尝试这样的事情（未经测试）：
[HttpPost]
public ActionResult Edit(FoodItem foodItem)
{
if (ModelState.IsValid)
{
int id = foodItem.Id;
// Load food item with related categories first
var item = context.FoodItems
.Include(f => f.Categories)
.Single(f => f.Id == id);
// Process changed scalar values
context.Entry(item).CurrentValues.SetValues(foodItem);
// Brute force processing of relations
// This can be optimized - instead of deleting all and adding all again
// you can manually compare which relations already exists, add new and
// remove non existing but let's make that as a homework
item.Categories.Clear();
var cList = Request["CategoryId"].Split(',');
foreach (var c in cList)
{
var ci = Convert.ToInt32(c);
// Use find - if category was already loaded in the first query, it will
// be reused without additional query to DB
var category = context.Categories.Find(ci);
// Now add category to attached food item to create new relation
item.Categories.Add(category);
}
context.SaveChanges();
return RedirectToAction("Index");
}
return View(foodItem);
}
这看起来效率很低但是因为你正在处理多对多关系，在视图中可以添加或删除关系，这是唯一的方法。原因是：
有关分离对象图和处理关系的更多信息是described here。它与ObjectContext API有关，但DbContext API只是包装，因此仍然存在相同的限制。
答案 1 :(得分：0)
除了Ladislav的答案，你可以使用http://haacked.com/archive/2008/10/23/model-binding-to-a-list.aspx删除Request []。split（）部分，这使得：
[HttpPost]
public ActionResult Edit(FoodItem foodItem, ICollection<int> CategoryId)
{
if (ModelState.IsValid)
{
int id = foodItem.Id;
// Load food item with related categories first
var item = context.FoodItems
.Include(f => f.Categories)
.Single(f => f.Id == id);
// Process changed scalar values
context.Entry(item).CurrentValues.SetValues(foodItem);
// Brute force processing of relations
// This can be optimized - instead of deleting all and adding all again
// you can manually compare which relations already exists, add new and
// remove non existing but let's make that as a homework
item.Categories.Clear();
foreach (var id in CategoryID)
{
// Use find - if category was already loaded in the first query, it will
// be reused without additional query to DB
var category = context.Categories.Find(id);
// Now add category to attached food item to create new relation
item.Categories.Add(category);
}
context.SaveChanges();
return RedirectToAction("Index");
}
return View(foodItem);
}