实体框架中可选的多对一或零问题

Question

我正在尝试创建一个快速的演示商店，但我失败了optional many to one or zero关系。

相关课程是：

1. 项目

   public class Item
   {
       public int ID { get; set; }
       public string Name { get; set; }
       public int SubCategoryID { get; set; }
       public virtual SubCategory Category { get; set; }
       public double Price { get; set; }
   }
   

2. 顺序

   public class Order
   {
       public int ID { get; set; }
       public DateTime DateOfOrder { get; set; }
       public virtual ICollection<Item> Items { get; set; }
       public int CustomerID { get; set; }
       public virtual Customer Customer { get; set; }
   }
   

然而，我感到困惑，因为查看模型显示：



但是，数据库本身显示（对于项目）：



这表明每件商品只能属于一个订单。

我是否必须创建一个包含多个订单/项目的单独类？

我似乎记得EF自动执行此操作，但是，我几年没有触及它，我只能记住以前的事情。

Answer 1

我不得不补充道：

00:00.0 Host bridge [0600]: Intel Corporation Core Processor DRAM Controller [8086:0044] (rev 12)
    Subsystem: Dell Device [1028:0434]
    Kernel driver in use: agpgart-intel
00:02.0 VGA compatible controller [0300]: Intel Corporation Core Processor Integrated Graphics Controller [8086:0046] (rev 12)
    Subsystem: Dell Device [1028:0434]
    Kernel driver in use: i915
00:16.0 Communication controller [0780]: Intel Corporation 5 Series/3400 Series Chipset HECI Controller [8086:3b64] (rev 06)
    Subsystem: Dell Device [1028:0434]
    Kernel driver in use: mei_me
00:1a.0 USB controller [0c03]: Intel Corporation 5 Series/3400 Series Chipset USB2 Enhanced Host Controller [8086:3b3c] (rev 06)
    Subsystem: Dell Device [1028:0434]
    Kernel driver in use: ehci-pci
00:1b.0 Audio device [0403]: Intel Corporation 5 Series/3400 Series Chipset High Definition Audio [8086:3b56] (rev 06)
    Subsystem: Dell Device [1028:0434]
    Kernel driver in use: snd_hda_intel
00:1c.0 PCI bridge [0604]: Intel Corporation 5 Series/3400 Series Chipset PCI Express Root Port 1 [8086:3b42] (rev 06)
    Kernel driver in use: pcieport
00:1c.5 PCI bridge [0604]: Intel Corporation 5 Series/3400 Series Chipset PCI Express Root Port 6 [8086:3b4c] (rev 06)
    Kernel driver in use: pcieport
00:1d.0 USB controller [0c03]: Intel Corporation 5 Series/3400 Series Chipset USB2 Enhanced Host Controller [8086:3b34] (rev 06)
    Subsystem: Dell Device [1028:0434]
    Kernel driver in use: ehci-pci
00:1e.0 PCI bridge [0604]: Intel Corporation 82801 Mobile PCI Bridge [8086:2448] (rev a6)
00:1f.0 ISA bridge [0601]: Intel Corporation Mobile 5 Series Chipset LPC Interface Controller [8086:3b09] (rev 06)
    Subsystem: Dell Device [1028:0434]
    Kernel driver in use: lpc_ich
00:1f.2 SATA controller [0106]: Intel Corporation 5 Series/3400 Series Chipset 4 port SATA AHCI Controller [8086:3b29] (rev 06)
    Subsystem: Dell Device [1028:0434]
    Kernel driver in use: ahci
00:1f.3 SMBus [0c05]: Intel Corporation 5 Series/3400 Series Chipset SMBus Controller [8086:3b30] (rev 06)
    Subsystem: Dell Device [1028:0434]
00:1f.6 Signal processing controller [1180]: Intel Corporation 5 Series/3400 Series Chipset Thermal Subsystem [8086:3b32] (rev 06)
    Subsystem: Dell Device [1028:0434]
    Kernel driver in use: intel ips
03:00.0 Ethernet controller [0200]: Realtek Semiconductor Co., Ltd. RTL8101E/RTL8102E PCI Express Fast Ethernet controller [10ec:8136] (rev 02)
    Subsystem: Dell Device [1028:0434]
    Kernel driver in use: r8169
ff:00.0 Host bridge [0600]: Intel Corporation Core Processor QuickPath Architecture Generic Non-core Registers [8086:2c62] (rev 02)
    Subsystem: Intel Corporation Device [8086:8086]
ff:00.1 Host bridge [0600]: Intel Corporation Core Processor QuickPath Architecture System Address Decoder [8086:2d01] (rev 02)
    Subsystem: Intel Corporation Device [8086:8086]
ff:02.0 Host bridge [0600]: Intel Corporation Core Processor QPI Link 0 [8086:2d10] (rev 02)
    Subsystem: Intel Corporation Device [8086:8086]
ff:02.1 Host bridge [0600]: Intel Corporation Core Processor QPI Physical 0 [8086:2d11] (rev 02)
    Subsystem: Intel Corporation Device [8086:8086]
ff:02.2 Host bridge [0600]: Intel Corporation Core Processor Reserved [8086:2d12] (rev 02)
    Subsystem: Intel Corporation Device [8086:8086]
ff:02.3 Host bridge [0600]: Intel Corporation Core Processor Reserved [8086:2d13] (rev 02)
    Subsystem: Intel Corporation Device [8086:8086]

到项目......我永远不会这样称呼它，但看起来这是EF建立这种关系所必需的。

我确信它过去更容易，所以，请将这个问题保持开放，以便有人能给出更好的答案！

Answer 2

如果您向Orders实体添加Item的集合，EF将隐式为您创建数据库上的junction table。在此page中，您可以找到有关如何配置多对多关系的更多信息。

当您需要添加其他列（排除要加入的表的两个键）时，通常会映射联结表。在这种情况下，您需要在表示联结表的实体与Order和Item之间创建两个一对多关系。有些人建议始终映射联结表，因为这样您将所有表表示为实体，并且您可以从联结表开始编写查询。您可以在link中找到有关此主题的有趣信息。但是，根据我的经验，在大多数情况下，您可以完美地工作，而无需明确地映射联结表。