用流利的nhibernate映射继承看起来有点过时了

Question

我的模型的一部分看起来像这样：

public class Person
{
    public virtual Guid Id { get; set; }
    public string Name { get; set; }
}

public class Employee : Person
{
    public virtual int EmployeeNumber { get; set; }
    public virtual Person Partner { get; set; }
}

为此生成的sql是：

create table [Person] (
   Id UNIQUEIDENTIFIER not null,
   Name NVARCHAR(255) null,
   Employee_id UNIQUEIDENTIFIER null,
   primary key (Id)
)

create table Employee (
   Person_id UNIQUEIDENTIFIER not null,
   EmployeeNumber INT null,
   Partner_Id UNIQUEIDENTIFIER null,
   primary key (Person_id)
)

为什么Person表中有Employee_id UNIQUEIDENTIFIER null,？