public class ProductCategory
    {
        [Key]
        public int Id { get; set; }

        [DisplayName("類別名稱")]
        [Required(ErrorMessage = "請輸入類別名稱")]
        [MaxLength(20, ErrorMessage = "類別名稱不可超過20個字")]
        public string CategoryName { get; set; }

        //其它省略

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


public class Product
    {
        [Key]
        public int Id { get; set; }

        [DisplayName("類別")]
        [Required]
        public virtual ProductCategory ProductCategory { get; set; }

        [DisplayName("名稱")]
        [Required(ErrorMessage = "請輸入名稱")]
        public string ProductName { get; set; }

        //其它省略

    }