fluent nhibernate映射到没有公开属性的私有/受保护字段

我有以下的 Person 和 Gender 类(我不是真的,但是为了理解我的观点,这个例子被简化了),使用nhibernate(fluent nhibernate)我想映射数据库列” GenderId “[int]值到受保护的int _genderId 我的领域 人 班级。我该怎么做?

仅供参考,映射和域对象在单独的程序集中。

public class Person : Entity
{
    protected int _genderId;

    public virtual int Id { get; private set; }
    public virtual string Name { get; private set; }

    public virtual Gender Gender
    {
        get { return Gender.FromId(_genderId); }
    }
}

public class Gender : EnumerationBase<Gender>
{
    public static Gender Male
        = new Gender(1, "Male");

    public static Gender Female
        = new Gender(2, "Female");

    private static readonly Gender[] _genders
        = new[] { Male, Female };

    private Gender(int id, string name)
    {
        Id = id;
        Name = name;
    }

    public int Id { get; private set; }
    public string Name { get; private set; }

    public static Gender FromId(int id)
    {
        return _genders.Where(x => x.Id == id).SingleOrDefault();
    }
}