NHibernate on a table with two "primary" keys - database

Question

I'm learning NHibernate in order to layer it over a rather peculiar legacy database. Other applications use the same live database, so I can't make changes that will affect them.
I've run into a problem because one table, which represents hardware devices, has two columns that are used as de facto primary keys. One is the real primary key, an auto-generated row id. The other is a unique and non-null hardware serial number.
Many other tables in the database have a foreign-key relationship with this table. However, some of them use the real primary key - the integer row id - as a foreign key, and some use the hardware id of the device instead.
Note that in practice a hardware ID and row ID, once paired, will remain paired.
Will I be able to create mappings to deal with this in NHibernate, or will I need to create some views to give me a more standardized schema, and use INSTEAD OF triggers to make them updatable?
The DB in use is MSSQL 2000, in case this makes a difference.

Answer 1

In your situation I would do the following:
public class HardwareDevice{
public virtual int Id {get; set;}
public virtual string SerialNumber {get; set;}
//Other stuff
}
public class DomainThingA {
public virtual int Id {get; set;}
public virtual HardwareDevice Device {get; set;}
//Other stuff
}
public class DomainThingB {
public virtual int Id {get; set;}
public virtual HardwareDevice Device {get; set;}
//Other stuff
}
Map out your HardwareDevice class using the AutoGenerated Id as the primary key.
My examples use FluentNhibernate for the class maps.
public class HardwareDeviceMap : ClassMap<HardwareDevice> {
public HardwareDeviceMap(){
Id(x=>x.Id).GeneratedBy.Native().Column("Id"); //Uses auto number
Map(x=>x.SerialNumber).Column("SerialNumber");
//Other mappings
}
}
Now for mapping out the other two classes:
public class DomainThingAMap : ClassMap<DomainThingA> {
public DomainThingAMap(){
Id(x=>x.Id).GeneratedBy.Native(); //Uses auto number
References(x=>x.Device)
.Column("DeviceId"); //Joins on Id in HardwareDevice Table by default
//Other mappings
}
}
public class DomainThingBMap : ClassMap<DomainThingB> {
public DomainThingBMap(){
Id(x=>x.Id).GeneratedBy.Native(); //Uses auto number
References(x=>x.Device)
.Column("SerialNumber") //Column in DomainThingB Table
.PropertyRef("SerialNumber"); //Joins using SerialNumber column (hardware device table)
//Other mappings
}
}
The Property-Ref feature of the class maps allows you to join on columns which are not the primary key for these types of legacy database purposes.