Question

我在我的项目中首先使用数据库实体框架5。我相信在外键虚拟字段中延迟加载值存在一些问题，我已经尝试了所有可能的方法，但到目前为止我无法解决问题。

以下是详细信息：

我有一个CustomerDiscountCardController索引方法，如下所示：

public ActionResult Index(String searchTerm = null, int page = 1)
        {
            var model = CustomerDiscountCardPagedList(searchTerm, page);


            ViewBag.CustomerID = new SelectList(db.Customers.OrderBy(c => c.FirstName), "CustomerID", "FirstName");
            if (Request.IsAjaxRequest())
            {
                return PartialView("_CustomerDiscountCard", model);
            }
            return View(model);
        }

        private IPagedList<CustomerDiscountCard> CustomerDiscountCardPagedList(String searchTerm, int page)
        {
            var model = db.CustomerDiscountCards.Include( d => d.DiscountCard).
                OrderBy(c => c.DiscountCard.CardName).
                Where(c => (c.DiscountCard.CardName.Contains(searchTerm))).
                Select(c => new
                {
                    DiscountCardID = c.DiscountCardID,
                    CustomerID = c.CustomerID,
                    IssueDate = c.IssueDate,
                    ExpiryDate = c.ExpiryDate,
                    CustomerDiscountCardID = c.CustomerDiscountCardID
                }
                ).ToList().Select(c => new CustomerDiscountCard
                {
                    DiscountCardID = c.DiscountCardID,
                    CustomerID = c.CustomerID,
                    IssueDate = c.IssueDate,
                    ExpiryDate = c.ExpiryDate,
                    CustomerDiscountCardID = c.CustomerDiscountCardID

                }).ToPagedList(page, 4);
            return model;
        }

我的数据上下文类如下所示：

    public OnlineFoodOrderingEntities2()
        : base("name=OnlineFoodOrderingEntities2")
    {
        this.Configuration.LazyLoadingEnabled = true;
        this.Configuration.ProxyCreationEnabled = true;
    }
    //Entities class code....

}

我自动生成的CustomerDiscountCard类如下所示：

public partial class CustomerDiscountCard
{
    public int CustomerDiscountCardID { get; set; }
    public Nullable<int> DiscountCardID { get; set; }
    public Nullable<int> CustomerID { get; set; }
    public Nullable<System.DateTime> IssueDate { get; set; }
    public Nullable<System.DateTime> ExpiryDate { get; set; }
    public virtual Customer Customer { get; set; }
    public virtual DiscountCard DiscountCard { get; set; }
}

现在的问题是在ajax请求上调用我的索引方法时返回_CustomerDiscountCard的部分布局

布局代码如下：

@using PagedList;
@using PagedList.Mvc;
@using OnlineFoodOrderingMVC.Models;
@model IPagedList<CustomerDiscountCard>

<div id="targetCustomerDiscountCardList">

    <div class="pagedList" data-ofo-target="#targetCustomerDiscountCardList">

        @Html.PagedListPager(Model, page => Url.Action("Index", "CustomerDiscountCard", new { page }),
        PagedListRenderOptions.MinimalWithItemCountText)
    </div>
    <div style="clear:both"></div>
    <div>
        <table class="entity_record_table">
            <tr class="entity_record_table_header">
                <th class="entity_record_table_header_item_text">
                    CustomerDiscountCards
                </th>
                <th class="entity_record_table_header_link_text">
                    Edit
                </th>
                <th class="entity_record_table_header_link_text">
                    View
                </th>
                <th class="entity_record_table_header_link_text">
                    Delete
                </th>
            </tr>
            @{
                String[] rowDisplay = { "entity_record_row_display1", "entity_record_row_display2" };
                String currentClass = "entity_record_row_display1";
                int count = 0;
            }

            @foreach (var item in Model)
            {

                count++;
                currentClass = rowDisplay[count % 2];

                <tr class="@(currentClass)">
                    <td class="entity_record_table_item">
                        @Html.DisplayFor(modelItem => item.DiscountCard.CardName)
                    </td>
                    <td class="entity_record_table_link">
                        @Ajax.ActionLink(" ", "Edit", new { id = item.CustomerDiscountCardID },
                        new AjaxOptions
                             {
                                 HttpMethod = "get",
                                 InsertionMode = InsertionMode.Replace,
                                 UpdateTargetId = "targetDiv"
                             }
                        , new { @class = "entity_record_edit" })
                    </td>
                    <td class="entity_record_table_link">
                        @Ajax.ActionLink(" ", "Details", new { id = item.CustomerDiscountCardID },
                              new AjaxOptions
                                {
                                    HttpMethod = "get",
                                    InsertionMode = InsertionMode.Replace,
                                    UpdateTargetId = "targetCustomerDiscountCardList"
                                }
                        , new { @class = "entity_record_view" })
                    </td>
                    <td class="entity_record_table_link">
                        @Ajax.ActionLink(" ", "Delete", new { id = item.CustomerDiscountCardID },
                              new AjaxOptions
                                {
                                    HttpMethod = "get",
                                    InsertionMode = InsertionMode.Replace,
                                    UpdateTargetId = "targetCustomerDiscountCardList"
                                }
                        , new { @class = "entity_record_delete" })
                    </td>
                </tr>

            }

        </table>
    </div>

</div>

所以当剃刀引擎渲染我的部分视图并执行以下命令时

item.DiscountCard.CardName

我得到DicountCard为null，我无法理解是什么导致这个为null可以任何人请帮助我这个。非常感谢。

Answer 1

问题是DiscountCard确实为空。在您提取项目时，您使用Select从值的子集中创建新类：

            Select(c => new
            {
                DiscountCardID = c.DiscountCardID,
                CustomerID = c.CustomerID,
                IssueDate = c.IssueDate,
                ExpiryDate = c.ExpiryDate,
                CustomerDiscountCardID = c.CustomerDiscountCardID
            }
            ).ToList().Select(c => new CustomerDiscountCard
            {
                DiscountCardID = c.DiscountCardID,
                CustomerID = c.CustomerID,
                IssueDate = c.IssueDate,
                ExpiryDate = c.ExpiryDate,
                CustomerDiscountCardID = c.CustomerDiscountCardID

            })

首先，你所做的是重复的。没有理由选择匿名对象，强制转换为列表，然后选择具有相同数据的真实类实例。只需直接选择类实例，即：

            Select(c => new CustomerDiscountCard
            {
                DiscountCardID = c.DiscountCardID,
                CustomerID = c.CustomerID,
                IssueDate = c.IssueDate,
                ExpiryDate = c.ExpiryDate,
                CustomerDiscountCardID = c.CustomerDiscountCardID

            })

其次，您从未为DiscountCard设置值，仅为DiscountCardId设置值。当您创建这样的新实例时，它不附加到您的上下文，并且无法获取或检索此相关值。这带给我们......

第三，你为什么一开始就选择任何东西？您从可枚举的CustomerDiscountCard开始，然后选择可枚举的新CustomerDiscountCard实例。这样做的唯一方法是打破这些项目对你的上下文的附件，有效地杀死任何延迟加载任何东西的能力。

<强>更新

只需将其更改为：

var model = db.CustomerDiscountCards.Include( d => d.DiscountCard).
            OrderBy(c => c.DiscountCard.CardName).
            Where(c => (c.DiscountCard.CardName.Contains(searchTerm))).
            ToPagedList(page, 4);

延迟加载不与Entity Framework 5.0一起使用的相关实体

1 个答案: