C# 实体框架6.1.2：零或一对多

下面的代码解析XML并将值存储到实体框架对象中，这些对象最终会存储到SQL数据库中：

Program.cs：

namespace locationProject
{
    class Program
    {
        static void Main(string[] args)
        {
            XDocument xml = XDocument.Load("locationDoc.xml");
            XNamespace cm = "http://somenamespace.com/cm";

            List<Location> locationColl = new List<Location>();
            List<Event> eventColl = new List<Event>();
            locationModelContainer context = new locationModelContainer();

            var i = 0;
            var i2 = 0;

            foreach (var continent in xml.Descendants(cm + "Continent"))
            {
                Location l = new Location();
                locationColl.Add(l);

                locationColl[i].Continent = (string)continent.Element(cm + "Name");

                foreach (var country in continent.Elements(cm + "Country"))
                {
                    locationColl[i].Country = (string)country.Element(cm + "Name");

                    foreach (var city in country.Elements(cm + "City"))
                    {
                        locationColl[i].City = (string)city.Element(cm + "Name");

                        context.Locations.Add(locationColl[i]);

                        foreach (var events in city.Elements(cm + "Event"))
                        {
                            Event e = new Event();
                            eventColl.Add(e);

                            eventColl[i2].Name = (string)events.Element(cm + "Name");
                            eventColl[i2].Date = (string)events.Element(cm + "Date");

                            context.Events.Add(eventColl[i2]);
                            i2++;
                        }
                        i++;
                    }
                }
            }
            context.SaveChanges();
        }
    }
}

使用一对多关系（一个位置对多个事件）可以如预期的那样工作。

Location\u LocationID

外键将填充相关的

LocationID

尽管如此，如上代码所示，我的预期设计是使用零或一对多关系（零或一个位置对多个事件）。但是，使用此关系不会填充

Location\u LocationID

外键，而是在保存到数据库时，所有

Location\u LocationID

字段都包含

NULL

---

有人能解释一下为什么他们不使用这种关系吗？提前谢谢

您当前未设置任何事件的位置。所以它当然总是空的。您也只在每个大陆创建一个位置。将创建位置的位置移动到枚举城市的位置

foreach (var continent in xml.Descendants(cm + "Continent"))
{
     var continent = (string)continent.Element(cm + "Name");

     foreach (var country in continent.Elements(cm + "Country"))
     {
         var country = (string)country.Element(cm + "Name");

         foreach (var city in country.Elements(cm + "City"))
         {
             Location l = new Location();
             l.Continent = continent;
             l.Country = country;
             l.City = (string)city.Element(cm + "Name");
             context.Locations.Add(l);
             locationColl.Add(l);

             foreach (var events in city.Elements(cm + "Event"))
             {
                 Event e = new Event();
                 eventColl.Add(e);

                 e.Location = l;
                 eventColl[i2].Name = (string)events.Element(cm + "Name");
                 eventColl[i2].Date = (string)events.Element(cm + "Date");

                 context.Events.Add(eventColl[i2]);
                 i2++;
             }
             i++;
         }
     }
 }
 context.SaveChanges();

---

谢谢你的回复。很抱歉问，但我如何才能做到这一点？@Cryptonaut我更新了答案。。。但后来我意识到你的密码有点坏了。你只会在每个大陆而不是每个城市找到一个位置。您在错误的点创建位置。

namespace locationProject
{
    using System;
    using System.Collections.Generic;

    public partial class Location
    {
        public Location()
        {
            this.Events = new HashSet<Event>();
        }

        public int LocationID { get; set; }
        public string Continent { get; set; }
        public string Country { get; set; }
        public string City { get; set; }

        public virtual ICollection<Event> Events { get; set; }
    }
}

    namespace locationProject
    {
        using System;
        using System.Collections.Generic;

        public partial class Event
        {
            public int EventID { get; set; }
            public Nullable<int> Location_LocationID { get; set; }
            public string Name { get; set; }
            public string Date { get; set; }

            public virtual Location Location { get; set; }
        }
    }

public partial class locationModelContainer : DbContext
{
    public locationModelContainer()
        : base("name=locationModelContainer")
    {
    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        throw new UnintentionalCodeFirstException();
    }

    public virtual DbSet<Location> Locations { get; set; }
    public virtual DbSet<Event> Events { get; set; }
    }
}

SET QUOTED_IDENTIFIER OFF;
GO
USE [Locations];
GO
IF SCHEMA_ID(N'dbo') IS NULL EXECUTE(N'CREATE SCHEMA [dbo]');
GO

IF OBJECT_ID(N'[dbo].[FK_LocationEvent]', 'F') IS NOT NULL
    ALTER TABLE [dbo].[Events] DROP CONSTRAINT [FK_LocationEvent];
GO

IF OBJECT_ID(N'[dbo].[Locations]', 'U') IS NOT NULL
    DROP TABLE [dbo].[Locations];
GO
IF OBJECT_ID(N'[dbo].[Events]', 'U') IS NOT NULL
    DROP TABLE [dbo].[Events];
GO

CREATE TABLE [dbo].[Locations] (
    [LocationID] int IDENTITY(1,1) NOT NULL,
    [Continent] nvarchar(max)  NOT NULL,
    [Country] nvarchar(max)  NOT NULL,
    [City] nvarchar(max)  NOT NULL
);
GO

CREATE TABLE [dbo].[Events] (
    [EventID] int IDENTITY(1,1) NOT NULL,
    [Location_LocationID] int  NULL,
    [Name] nvarchar(max)  NOT NULL,
    [Date] nvarchar(max)  NOT NULL
);
GO

ALTER TABLE [dbo].[Locations]
ADD CONSTRAINT [PK_Locations]
    PRIMARY KEY CLUSTERED ([LocationID] ASC);
GO

ALTER TABLE [dbo].[Events]
ADD CONSTRAINT [PK_Events]
    PRIMARY KEY CLUSTERED ([EventID] ASC);
GO

ALTER TABLE [dbo].[Events]
ADD CONSTRAINT [FK_LocationEvent]
    FOREIGN KEY ([Location_LocationID])
    REFERENCES [dbo].[Locations]
        ([LocationID])
    ON DELETE NO ACTION ON UPDATE NO ACTION;
GO

CREATE INDEX [IX_FK_LocationEvent]
ON [dbo].[Events]
    ([Location_LocationID]);
GO

foreach (var continent in xml.Descendants(cm + "Continent"))
{
     var continent = (string)continent.Element(cm + "Name");

     foreach (var country in continent.Elements(cm + "Country"))
     {
         var country = (string)country.Element(cm + "Name");

         foreach (var city in country.Elements(cm + "City"))
         {
             Location l = new Location();
             l.Continent = continent;
             l.Country = country;
             l.City = (string)city.Element(cm + "Name");
             context.Locations.Add(l);
             locationColl.Add(l);

             foreach (var events in city.Elements(cm + "Event"))
             {
                 Event e = new Event();
                 eventColl.Add(e);

                 e.Location = l;
                 eventColl[i2].Name = (string)events.Element(cm + "Name");
                 eventColl[i2].Date = (string)events.Element(cm + "Date");

                 context.Events.Add(eventColl[i2]);
                 i2++;
             }
             i++;
         }
     }
 }
 context.SaveChanges();