使用具有自己的外部/导航键的Model类 [英] Working with a Model class that has a foreign/navigation key to itself

问题描述

我正在尝试在ASP.NET MVC 3中开发一个目录项目，并使用EF代码首先使用现有的数据库。我的数据库中有一个 Categories 表指向自己。为此，我写了下面的模型类。

  public class Category 
 {
 public int CategoryID {get;组; } 
 public string CategoryName {get;组; } 
 public int？ ParentCategoryID {get;组; } 
 public string CategoryDe​​sc {get;组; } 
 
 [ForeignKey（ParentCategoryID）] 
 public virtual Category ParentCategory {get;组; } 
 public virtual ICollection< Product>产品{get;组; } 
} 
  

问题：我无法理解为我如何与这个课程一起工作。在使用和传递以下代码到视图

var cat = dbStore.Categories.Include（ParentCategory）。ToList（） code>。

我收到此错误：对象引用未设置为对象的实例。这是因为根类别具有null ParentCategoryID。请告诉我如何使用这个代码或任何可以帮助我理解在这种情况下工作的资源。只要任何类型的代码将有助于使用上述模型，如显示列表或菜单或任何东西，只是任何东西。

解决方案

通常你做的是从顶级类别到底级类别。为了做到这一点，你需要在你的课上定义 SubCategories 集合

  public class Category 
 {
 public int CategoryID {get;组; } 
 public string CategoryName {get;组; } 
 public int？ ParentCategoryID {get;组; } 
 public string CategoryDe​​sc {get;组; } 
 
 [ForeignKey（ParentCategoryID）] 
 public virtual Category ParentCategory {get;组; } 
 
 [InverseProperty（ParentCategory）] 
 public virtual ICollection< Category>子类{get;组; } 
 
 public virtual ICollection< Product>产品{get;组; 
} 
  

然后，您检索顶级类别

  var topCategories = dbStore.Categories 
 .Where（category => category.ParentCategoryID == null）
 .Include（category => ; category.SubCategories）.ToList（）; 
  

之后，您可以遍历hierachey

  foreach（top top类别中的var topCategory）
 {
 //使用顶级类别
 foreach（topCategory.SubCategories中的var子类别）
 {
 
} 
 
} 
  

I am trying to develop a catalog project in ASP.NET MVC 3 and using EF Code first with an existing Database. There is a Categories table in my database that points to itself. For that, I have written the following model class. --"Correct me if the model is wrong"--

public class Category
{
    public int CategoryID { get; set; }
    public string CategoryName { get; set; }
    public int? ParentCategoryID { get; set; }
    public string CategoryDesc { get; set; }

    [ForeignKey("ParentCategoryID")]
    public virtual Category ParentCategory { get; set; }
    public virtual ICollection<Product> Products { get; set; }
}

Question : I am unable to understand as to how can i work with this class. While using and passing the following code to the view

var cat = dbStore.Categories.Include("ParentCategory").ToList().

I got this error : Object reference not set to an instance of an object. This is happening because the root category has null ParentCategoryID. Please tell me how will you work with this code or any resource that can help me understand working in such scenarios. Just any sort of code will be helpful that uses the above the model, like displaying a list or a menu or anything, just anything.

解决方案

Usually what you do is travel from top level categories to bottom level categories. Inorder to do that first you need to define SubCategories collection in your class

public class Category
{
    public int CategoryID { get; set; }
    public string CategoryName { get; set; }
    public int? ParentCategoryID { get; set; }
    public string CategoryDesc { get; set; }

    [ForeignKey("ParentCategoryID")]
    public virtual Category ParentCategory { get; set; }

    [InverseProperty("ParentCategory")]
    public virtual ICollection<Category> SubCategories{ get; set; }

    public virtual ICollection<Product> Products { get; set; }
}

Then you retrieve the top level categories

var topCategories = dbStore.Categories
   .Where(category => category.ParentCategoryID == null)
   .Include(category => category.SubCategories).ToList();

After that you can traverse the hierachey

foreach(var topCategory in topCategories)
{
    //use top category
    foreach(var subCategory in topCategory.SubCategories)
    {

    }

}

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