急于加载ASP MVC从导航属性返回null

Question

我有一个启用了代码优先迁移的ASP MVC 5 Web应用程序。我有两个名为Post.cs和PostSubject.cs的表/类。

Post.cs

public class Post
{
    public int Id { get; set; }

    [Required(ErrorMessage="Title is required")]
    [MaxLength(255,ErrorMessage="Title too long")]
    public string Title { get; set; }

    [MaxLength(1023,ErrorMessage="Description too long")]
    [Required(ErrorMessage = "Description is required")]
    public string Description { get; set; }

    public PostSubject PostSubject { get; set; }


    [Display(Name="Stream")]
    [Required(ErrorMessage="Choose a Stream")]
    public int PostSubjectId { get; set; }


    [NotMapped]
    public HttpPostedFileBase Attachment { get; set; }
}

PostSubject.cs

public class PostSubject
{
    public short Id { get; set; }
    public string SubjectName { get; set; }
}

我正在尝试将所有帖子及其PostSubjects加载到索引视图中。

索引操作

[HttpGet]
public ActionResult Index()
{
   var contributions =_context.Posts.Include(m=>m.PostSubject).ToList();
   return View(contributions);
}

索引HTML

@model IEnumerable<Cluster_University.Models.Post>

@{
    ViewBag.Title = "Index";
    Layout = "~/Views/Shared/_Layout.cshtml";
}


<table>
    <tr>
        <td>Title</td>
        <td>Descripttion</td>
        <td>Subject</td>
    </tr>
    @foreach (var posts in Model)
    {
        <tr>
            <td>@posts.Title</td>
            <td>@posts.Description</td>
            <td>@posts.PostSubject.SubjectName</td>
        </tr>
    }
</table>

问题：@ Posts.PostSubject.SubjectName出现空引用错误

请让我理解代码中的问题。