fluent nHibernate mapping

public class Permit
    {
    public int PermitId { get; set; }
    public char Discipline { get; set; }
    public PermitDetails PermitDetails { get; set; }
    }

public PermitDetails
    {
    public int PermitId { get; set; }
    }

public class GasPermitDetails : PermitDetails
    {
       ... permit details
    }

public class ElectricalPermitDetails : PermitDetails
    {
       ... permit details
    }

*tblPermit*
PermitId, int
Discipline, char
.... some other columns

*tblGas*
PermitId, int
....gasDetails

*tblElectrical*
PermitId, int
....electrical details

+---------------+         +----------------------+
|               | 1     1 |                      |
|   tblPermit   |<------->| tblGas               |
|               |         | (for gas discipline) |
|               |         |                      |
+---------------+         +----------------------+
|               |         |                      |
| {PK} PermitId |         | {FK} PermitID        | <- ForeignKey to tblPermit.PermitID
|               |         |                      |
+---------------+         +----------------------+