c# - 多对多关系基本示例 (MVC3)

Question

我有一个 MVC3 C# 项目，我有一个 FoodItem 和 FoodItemCategory 模型。两种模型如下图所示:

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 _CreateOrEdit.cshtml View ，它最初是从 Scaffolder 生成的，我修改它以包含所有类别并选中食品所属的框。一个食品项目可以有许多或所有的类别。 View 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)

如您所见，我有一个嵌套循环，它为每个类别创建一个复选框，在创建每个类别时，我循环遍历并在我的模型的 Categories 属性中检查该特定类别。如果它存在，我设置复选框的 checked 属性。如果您选中一个框并单击保存，在 Controller 上的 HttpPost 操作中，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);
    }

我可以一次保存类别。如果我返回 View 并单击保存，我会收到以下错误:

A duplicate value cannot be inserted into a unique index. [ Table name = >FoodItemCategoryFoodItems,Constraint name = PK_FoodItemCategoryFoodItems_00000000000000A8 ] Description: An unhandled exception occurred during the execution of the current web request. Please >review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Data.SqlServerCe.SqlCeException: A duplicate value cannot be inserted into >a unique index. [ Table name = FoodItemCategoryFoodItems,Constraint name = >PK_FoodItemCategoryFoodItems_00000000000000A8 ]

Source Error:

Line 97: context.Entry(foodItem).State = EntityState.Modified; Line 98: foodItem.Categories = categories; Line 99: context.SaveChanges(); Line 100: return RedirectToAction("Index"); Line 101: }

不确定这是否重要，但我使用的是 SQLServer Compact Edition 4。我这样做是否正确？像这样的事情的正常编码实践是什么？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都使用相同的关系模型。

Answer 1

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，但因为您处理的是多对多关系，其中可以在 View 中添加或删除关系，所以这是唯一的方法。原因是:

• 你必须说 EF 哪些关系被添加，哪些被删除
• 如果您只是简单地添加所有相关类别，您将再次插入关系
• 您不能说 EF 删除了哪些关系，因为您不传输有关未检查类别的信息

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 只是对其的包装，因此仍然存在相同的限制。