嗨!我在两个表之间有一些关系问题，一对多。

整个概念如下:
表1:国家:除了它自己的字段之外，它还包含两个iccollections，其中一个指向地区集合，另一个指向城市集合。

表2:区域除了它自己的字段之外，它还包含一个CountryId和一个城市集合。

表3:城市除了它自己的字段之外，它还包含一个CountryId和一个RegionId(其中RegionId可以为空)。

其主要思想是，一个国家可以有地区或城市。这意味着从地区/城市到CountryId的每个外键不能为空。但是，从City到Region的外键允许为空，因为"一些"国家(在本例中是Region)是不必要的。

注意，这三个表还使用一个iccollection引用一个中间表，而中间表又存储它们之间的关系。

国家实体:

public class Country
{
    public Guid Id { get; set; }
    public int CountryId { get; set; }
    public Guid ComponentId { get; set; }
    public string NumCode { get; set; }
    public string Code2 { get; set; }
    public string Code3 { get; set; }
    public virtual ICollection<Region> Regions { get; set; }
    public virtual ICollection<City> Cities { get; set; }
    public virtual ICollection<AdvertGeography> AdvertsGeographies { get; set; }
}
public class CountryMap()
    {
        // Primary Key
        HasKey(t => t.Id);
        Property(e => e.Id).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
        // Unique Index
        Property(e => e.ComponentId)
            .HasColumnAnnotation("Index", new IndexAnnotation(new IndexAttribute("IX_Unique_Component", 1) { IsUnique = true }));
        Property(e => e.CountryId)
            .HasColumnAnnotation("Index", new IndexAnnotation(new IndexAttribute("IX_Unique_Country", 1) { IsUnique = true }));
        // Properties
        // Table & Column Mappings
        ToTable("Country");
        Property(e => e.Id).HasColumnName("Id");
        Property(e => e.CountryId).HasColumnName("CountryId");
        Property(e => e.ComponentId).HasColumnName("ComponentId");
        Property(e => e.Code2).HasColumnName("Code2");
        Property(e => e.Code3).HasColumnName("Code3");
        Property(e => e.NumCode).HasColumnName("NumCode");
        // Relationships
        HasMany(t => t.Regions)
            .WithRequired(t => t.Country).WillCascadeOnDelete(false);
        HasMany(t => t.Cities)
            .WithRequired(t => t.Country).WillCascadeOnDelete(false);
    }

区域实体:

 public class Region
{
    public Region()
    {
        this.Cities = new HashSet<City>();
    }
    public Guid Id { get; set; }
    public Guid CountryId { get; set; }
    public virtual Country Country { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Office> Offices { get; set; }
    public virtual ICollection<City> Cities { get; set; }
    public virtual ICollection<OpenApplication> OpenApplications { get; set; }
    public virtual ICollection<Subscription> Subscriptions { get; set; }
    public virtual ICollection<AdvertGeography> AdvertsGeographies { get; set; }
}
public class RegionMap()
    {
        // Primary Key
        HasKey(t => t.Id);
        Property(e => e.Id).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
        // Unique Index
        Property(e => e.Name)
            .HasColumnAnnotation("Index", new IndexAnnotation(new IndexAttribute("IX_Unique", 1) { IsUnique = true }));
        Property(e => e.CountryId)
            .HasColumnAnnotation("Index", new IndexAnnotation(new IndexAttribute("IX_Unique", 2) { IsUnique = true }));
        // Properties
        Property(e => e.Name).IsRequired().HasMaxLength(512);
        // Table & Column Mappings
        ToTable("Region");
        Property(e => e.Id).HasColumnName("Id");
        Property(e => e.Name).HasColumnName("Name");
        Property(e => e.CountryId).HasColumnName("CountryId");
        // Relationships
        HasMany(t => t.Subscriptions)
            .WithMany(t => t.Regions);
        HasMany(t => t.OpenApplications)
            .WithMany(t => t.Regions);
        HasMany(t => t.Offices)
            .WithRequired(t => t.Region);
        HasMany(t => t.Cities)
            .WithRequired(t => t.Region);
        HasRequired(t => t.Country).WithMany(t => t.Regions).WillCascadeOnDelete(false);
    }

城市实体:

public class City
{
    public Guid Id { get; set; }
    public Guid CountryId { get; set; }
    public virtual Country Country { get; set; }
    public Guid? RegionId { get; set; }
    public virtual Region Region { get; set; }
    public string Name { get; set; }
    public virtual ICollection<AdvertGeography> AdvertsGeographies { get; set; }
}
public class CityMap()
    {
        //Primary Key
        HasKey(e => e.Id);
        Property(e => e.Id).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
        //Unique Index
        Property(e => e.Name)
            .HasColumnAnnotation("Index", new IndexAnnotation(new IndexAttribute("IX_Unique", 1) { IsUnique = true }));

        //Properties
        Property(e => e.Name).IsRequired().HasMaxLength(512);
        //Table & Column Mappings
        ToTable("City");
        Property(e => e.Id).HasColumnName("Id");
        Property(e => e.Name).HasColumnName("Name");
        Property(e => e.CountryId).HasColumnName("CountryId");
        Property(e => e.RegionId).HasColumnName("RegionId");
        // Relationships
        HasRequired(t => t.Country).WithMany(t => t.Cities).WillCascadeOnDelete(false);
        HasOptional(t => t.Region).WithMany(t => t.Cities).HasForeignKey(t => t.RegionId);
    }    
/J

有人知道吗?很高兴得到任何帮助