asp.NET MVC 丢失引用字段的值

Question

我有课

public partial class Advertisement
{
    public int AdvertisementId { get; set; }
    public string Description { get; set; }
    public City CityIdFrom { get; set; }
    public City CityIdTo { get; set; }
    public int Weight { get; set; }
}

代表表格广告。类城市也是如此

public class City
{
    public Int32 CityId { get; set; }
    public string Name { get; set; }
}

现在我有一个名为 View1Controller 的视图控制器，它有

DbConnection db = new DbConnection();

public ActionResult Index()
{
    var query = from item in db.Advertisements.AsEnumerable() select item;

    return View(query);
}

最后还有一个View1.cshtml文件

@model IEnumerable<MvcApplication1.Models.Advertisement>

@{
    ViewBag.Title = "View1";
}

<h2>View1</h2>

<table>
@foreach(var item in Model)
{
    <tr>
        <td>@item.Description</td>
        <td>@item.CityIdFrom</td>
    </tr>
}
</table>

我查看了 SQL Profiler 并查看了生成的查询

SELECT 
[Extent1].[AdvertisementId] AS [AdvertisementId], 
[Extent1].[Description] AS [Description], 
[Extent1].[Weight] AS [Weight], 
[Extent1].[CityIdFrom_CityId] AS [CityIdFrom_CityId], 
[Extent1].[CityIdTo_CityId] AS [CityIdTo_CityId]
FROM [dbo].[Advertisements] AS [Extent1]

并执行我得到的查询：

2   None    5000    1   2
3   Test    1000    3   4

但是，当查询被命中时，由于某种原因，CityIdFrom 和 CityIdTo 都为空。因此结果表看起来

None 
Test 

而不是预期

None 1
Test 3

我做错了什么？

Answer 1

您需要为 CityFrom 和 CityTo 添加一个 Include，或者您可以将这些引用的实体设为虚拟。选项 1（包含）避免了 ORM 中常见的 select n+1 问题。

var query = db.Advertisements.Include("CityIdFrom").Include("CityIdTo").AsEnumerable();
return View(query);

Answer 2

如果您使用实体框架，则需要将属性设为虚拟。

public partial class Advertisement
{
    public int AdvertisementId { get; set; }
    public string Description { get; set; }
    public virtual City CityIdFrom { get; set; }
    public virtual City CityIdTo { get; set; }
    public int Weight { get; set; }
}