如何用实体框架描述同一表中记录之间的关系

Question

在EF中，我需要描述以下关系：

一家公司可能有很多地点，比如

headquarter         <= main location 
      plant      ----+
      warehouse      |
      store-1        +----> child Locations
      store-2        |
      store-n    ----+

所以我需要一个位置模型中的mainLocationID，这样我就可以

1)给定主位置，我可以访问它的所有子位置

2)给定一个孩子的位置，我可以找到它的主要位置。

所以我试着做了以下几件事：

public class Location
{
    public int id { get; set; }
    public string name { get; set; }
    public string type { get; set; }
    public bool flagMainLocation { get; set; }

    public int? mainLocationID { get; set; }
    public virtual ICollection<Location> ChildLocations { get; set; }
}

在我的db上下文中

public class myappContext : DbContext
{
    public myappContext() : base("myappContext")
    {
    }

    public DbSet<Location> Locations { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();

        modelBuilder.Entity<Location>()
                            .HasOptional(l => l.ChildLocations)
                            .WithMany()
                            .HasForeignKey(l => l.mainLocationID);
    }
}

现在我陷入困境，因为当我试图为Location类搭建一个控制器时，我会得到以下错误

"myapp.DAL.Location_ChildLocations:: Multiplicity conflicts with 
the referential constraint in Role 'Location_ChildLocations_Target' 
in relationship 'Location_ChildLocations'. Because all of the 
properties in the Dependant Role are non-nullable, multiplicity 
of the Principal Role must be '1'."

我对EF不够精通，无法解密这条错误信息。有人能告诉我这种配置有什么问题吗？

我也希望能以这种方式获得主要位置。

Location myChildLocation = db.Locations.Find(some_location_id);
Location mainLocation = myChildLocation.mainLocation;

Answer 1

如果你试一试：

modelBuilder.Entity<Location>()
    .HasMany(l => l.ChildLocations)
    .WithOptional()
    .HasForeignKey(l => l.mainLocationID);

Answer 2

好的，在BiffBaffBoff的提示下，为了到达主位置，添加了一些语法糖，我终于让它运行了：

public class Location
{
    public int id { get; set; }
    public string name { get; set; }
    public string type { get; set; }
    public bool flagMainLocation { get; set; }

    public int? mainLocationID { get; set; }
    public virtual ICollection<Location> ChildLocations { get; set; }
    public virtual Location mainLocation { get; set;}
}

public class myappContext : DbContext
{
    public myappContext() : base("myappContext")
    {
    }

    public DbSet<Location> Locations { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();

        modelBuilder.Entity<Location>()
            .HasMany(l => l.ChildLocations)
            .WithOptional()
            .HasForeignKey(l => l.mainLocationID);
    }
}

参见github https://github.com/kranz/selfRefModel上的工作示例