fluent nHibernate mapping

Question

I am working on a legacy system and have introduced fluent nHibernate but have one last mapping that I can not seem to get working.

Code:

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
    }

Schema:

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

*tblGas*
PermitId, int
....gasDetails

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

If tblPermit.Discipline is "G", we need to get the data out of tblGas. If tblPermit.Discipline is "E", we need to get the data out of tblElectrical. I have messing around trying to get this figured out but have not had any luck so far.

score 0 · Answer 1 · answered Dec 27 '08 at 21:54

It looks to me that the schema came out right. You may need to be more specific about the problem you have because I take it that the PermitId for all the PermitDetails subclasses becomes foreign keys to the corresponding relation tables.

Eg. for the discipline "gas", the permit in question should have gas information for the permit. It has the relationship if the PermitID in tblGas is a foreign key to tblPermit's id. Like this:

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

You won't need the Discipline column to select the correct data. You only need one query that looks up tblElectrical, tblGas on the permit's identifier. That way, you'll get your permit along with it's information that way.

(PS. Excuse me for the use of UML class diagram to visualise the database tables, but crows feet notation does not translate well in ASCII)

Your diagram is correct. I just don't know how to write the mapping to tell nHibernate which type of data to load when it hydrates the data back. — Kudos2u2, Dec 27 '08 at 23:28
I don't know really how to do that myself (hibernate has gone off my radar recently). You could always check out http://www.hibernate.org/hib_docs/nhibernate/html/inheritance.html and http://stackoverflow.com/questions/296834/fluent-nhibernate-how-to-map-a-subclass-one-to-one if it gives any ideas. — Spoike, Jan 01 '09 at 14:12

score 0 · Answer 2 · answered Dec 28 '08 at 10:13

(if tblPermit.Discipline is "G" we need to get the data out of tblGas. if tblPermit.Discipline is "E" we need to get the data out of tblElectrical). I have messing around trying to get this figured out but have not had any luck so far.

The discriminator field is irrelevant, as the PK fields of the subtypes are FK's to their supertype's table anyway. So this is a table-per-subtype mapping.

fluent nHibernate mapping

2 Answers2