c# – Entity Framework 5 codefirst /必需和可选的外键关系为nu

但是当我尝试获取产品时,它的品牌和类别字段为空.类别是可选的,但品牌不是.所以至少必须设置品牌领域.我试过下面的代码.有什么我想念的吗？

public DbSet<Brand> Brands { get; set; }
    public DbSet<Product> Products { get; set; }
    public DbSet<Category> Categories { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Brand>()
            .HasMany(b => b.Products)
            .WithRequired(p => p.Brand)
            .HasForeignKey(p => p.BrandId);

        modelBuilder.Entity<Category>()
            .HasMany(c => c.Products)
            .WithOptional(p => p.Category)
            .HasForeignKey(p => p.CategoryId);
    }

在MVC控制器方面：

using (var db = new InonovaContext())
    {
        var product = db.Products.Single(p => p.Id == id);
        model.Description = product.Description;
        model.ImageUrl = product.ImageUrl;
        model.Name = product.Name;
        model.BreadCrumb = product.Brand.Name + " / " + product.Category == null ? "" : (product.Category.Name + " / ") + product.Name; // Here Brand and Category are null
    }

产品类如下

public class Product
{
    public int Id { get; set; }
    public int BrandId { get; set; }
    public virtual Brand Brand { get; set; }
    public string Name { get; set; }
    public int? CategoryId { get; set; }
    public virtual Category Category { get; set; }
    public string ImageUrl { get; set; }
    public string Description { get; set; }
}

品牌类如下：

public class Brand
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string ThumbLogoImageUrl { get; set; }
    public string Description { get; set; }
    public ICollection<Product> Products { get; set; }
}

谢谢.