实体框架不保存对象属性

Question

我正在开发使用ASP.Net MVC 4框架的在线商店。

我有一个对象类别 - 在存放物品的类别（男士服装 - 例如）：

public class Category 
{ 
    [Required] 
    public long id { get; set; } 

    [Required] 
    public string name { get; set; } 

    public bool active { get; set; } 
    public bool displayOnTop { get; set; } 
    public bool showSubcategoriesItems { get; set; } 

    public Category parentCategory { get; set; } 

    public virtual Collection<Item> items { get; set; } 

    public Category() 
    { 
     this.items = new Collection<Item>(); 
    } 
} 

有一个属性parentCategory - 这意味着这个类别包括到另一个（男士服装>例如衬衫）。

对于编辑现有的类别I使用了以下行动：

[HttpPost] 
public ActionResult EditCategory(Category editedCategory, string parentCategorySelector) 
{ 
    UsersContext db = new UsersContext(); 

    Category existingCategory = db.categories.SingleOrDefault(c => c.id == editedCategory.id); 
    long parentCategoryId = (long)Int32.Parse(parentCategorySelector); 
    Category parentCategory = db.categories.SingleOrDefault(c => c.id == parentCategoryId); 

    if (existingCategory == null) return RedirectToAction("Index", "Admin"); 

    existingCategory.name = editedCategory.name; 
    existingCategory.active = editedCategory.active; 
    existingCategory.displayOnTop = editedCategory.displayOnTop; 
    existingCategory.showSubcategoriesItems = editedCategory.showSubcategoriesItems; 
    existingCategory.parentCategory = parentCategory; 

    db.Entry(existingCategory.parentCategory).State = EntityState.Modified; 
    db.SaveChanges(); 

    return View(existingCategory); 
} 

一切工作正常，除非我想parentCategory设置为null - 它设置为NULL代码，但空值没有被保存到数据库 - 之前的值仍然存在。

但是，如果我不喜欢这样写道：

[HttpPost] 
public ActionResult EditCategory(Category editedCategory, string parentCategorySelector) 
{ 
    UsersContext db = new UsersContext(); 

    Category existingCategory = db.categories.SingleOrDefault(c => c.id == editedCategory.id); 
    long parentCategoryId = (long)Int32.Parse(parentCategorySelector); 
    Category parentCategory = db.categories.SingleOrDefault(c => c.id == parentCategoryId); 

    if (existingCategory == null) return RedirectToAction("Index", "Admin"); 

    existingCategory.name = editedCategory.name; 
    existingCategory.active = editedCategory.active; 
    existingCategory.displayOnTop = editedCategory.displayOnTop; 
    existingCategory.showSubcategoriesItems = editedCategory.showSubcategoriesItems; 
    existingCategory.parentCategory = null; 

    db.Entry(existingCategory.parentCategory).State = EntityState.Modified; 
    db.SaveChanges(); 

    return View(existingCategory); 
} 

即我将它设置为null无条件，它被正确保存。

我在做什么错？

Answer 1

为什么在类别下没有Key属性。 id？如果id是主键，它必须在那里。

您应该使用包括到load relate entities：

[HttpPost] 
    public ActionResult EditCategory(Category editedCategory, string parentCategorySelector) 
    { 
     using(UsersContext db = new UsersContext()) 
     { 
      Category existingCategory = db.categories.Include(c => c.parentCategory).SingleOrDefault(c => c.id == editedCategory.id); 
      long parentCategoryId = (long)Int32.Parse(parentCategorySelector); 
      Category parentCategory = db.categories.SingleOrDefault(c => c.id == parentCategoryId); 

      if (existingCategory == null) return RedirectToAction("Index", "Admin"); 

      existingCategory.name = editedCategory.name; 
      existingCategory.active = editedCategory.active; 
      existingCategory.displayOnTop = editedCategory.displayOnTop; 
      existingCategory.showSubcategoriesItems = editedCategory.showSubcategoriesItems; 
      existingCategory.parentCategory = parentCategory; 

      db.SaveChanges(); 

      return View(existingCategory); 
     } 
    } 

请注意，怎样的DbContext妥善处理。