Example model:

public interface IHierarchy<T> : IEntity 
    where T : Entity
{
    [Field]
    T Parent { get; set; }
}

[Serializable]
[HierarchyRoot]
public class MyEntity : Entity, IHierarchy<MyEntity>
{
    [Field, Key]
    public int Id { get; private set; }

    [Field(Length = 100)]
    public string Text { get; set; }

    public MyEntity()
        : base()
    {
    }

    public MyEntity(Session session)
        : base(session)
    {
    }

    [Field]
    public MyEntity Parent { get; set; }
}

On property Parent there is no FK in database (MSSQL)

DO version: 4.4.0.7460