为什么在这个例子中我不能访问 CategoryName？

Question

考虑以下类：

public class Pie
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int CategoryId { get; set; }
    public virtual Category Category { get; set; }
}

public class Category
{
    public int CategoryId { get; set; }
    public string CategoryName { get; set; }
    public string Description { get; set; }
    public ICollection<Pie> Pies { get; set; }
}

我有以下来自硬编码文件的数据：

public class MockPieRepository : IPieRepository
{
    private readonly ICategoryRepository _categoryRepository = new MockCategoryRepository();

    private List<Pie> pies = new List<Pie>() {
        new Pie { Id = 1, Name = "Apple Pie", Price = 12.95M, IsPieOfTheWeek = true, ShortDescription = "Yummy Apple Pie", LongDescription = "Lengthy apple pie", ImageThumbnailUrl = "images/thumbs/apple.jpg", ImageUrl = "images/apple.jpg", CategoryId = 1},
        new Pie { Id = 2, Name = "Rhubarb Pie", Price = 11.95M, IsPieOfTheWeek = false, ShortDescription = "Yummy Rhubarb Pie", LongDescription = "Lengthy Rhubarb pie", ImageThumbnailUrl = "images/thumbs/Rhubarb.jpg", ImageUrl = "images/Rhubarb.jpg", CategoryId = 1},
        new Pie { Id = 3, Name = "Cheesecake", Price = 8.95M, IsPieOfTheWeek = false, ShortDescription = "Yummy Cheesecake Pie", LongDescription = "Lengthy Cheesecake pie", ImageThumbnailUrl = "images/thumbs/Cheesecake.jpg", ImageUrl = "images/Cheesecake.jpg", CategoryId = 2},
        new Pie { Id = 4, Name = "Chocolate Cake", Price = 4.95M, IsPieOfTheWeek = false, ShortDescription = "Yummy Chocolate Pie", LongDescription = "Lengthy Chocolate pie", ImageThumbnailUrl = "images/thumbs/Chocolate.jpg", ImageUrl = "images/Chocolate.jpg", CategoryId = 3},
        new Pie { Id = 5, Name = "Mince Beef", Price = 6.99M, IsPieOfTheWeek = false, ShortDescription = "Yummy Mince Beef Pie", LongDescription = "Lengthy Mince Beef pie", ImageThumbnailUrl = "images/thumbs/Beef.jpg", ImageUrl = "images/Beef.jpg", CategoryId = 3}
    };

    public IEnumerable<Pie> GetPies()
    {
        return pies.OrderBy(p => p.Id);
    }
    public Pie GetPieById(int pieId)
    {
        return pies.FirstOrDefault(p => p.Id == pieId);
    }


}

public class MockCategoryRepository : ICategoryRepository
{
    public IEnumerable<Category> Categories {
        get {
            return new List<Category>
            {
                new Category { CategoryId = 1, CategoryName = "Fruit Pies", Description = "All fruity pies"},
                new Category { CategoryId = 2, CategoryName = "Cheesecakes", Description = "Cheesecakes to make your heart sing"},
                new Category { CategoryId = 3, CategoryName = "Seasonal Pies", Description = "Christmas, Halloween or Spring"}
            };
        }
    }
}

当我尝试从 @pie.Category.CategoryName 获取 CategoryName 时，该值为 null。显然我在尝试从 Pie 导航到 Category 时做错了，但谁能告诉我它是什么？

谢谢。

Answer 1

你永远不会设置pie.Category = category。

EF 在加载数据时会自动执行的一件事是，它将“解析”已在数据中检测到的导航属性。

如果您保存pie 并设置Category，则该数据将持久保存在数据库中，并且下次加载pie 时，您还将加载pie.Category。由于您尚未从数据库中存储（和检索）数据，因此没有发生这种情况。