Inheritance mapping in NHibernate with different named Id columns

Question

I've got three tables which share a lot of columns, so I thought to use inheritance to map them and make my data layer a bit more OOP, DRY and so on. In all of the three strategies for inheritance mapping mentioned on NHibernate manual (table per class hierarchy, table per subclass and table per concrete class) the superclass (or common interface) is mapped with an Id that is common to all derived subclasses. BUT none of the primary keys columns on my tables are called the same. Any idea how could I map this?

In a diagram:

TAXTYPE_1

clcpnd -> Differently named Id column
clcnmb -> Common but differently named column
clcprc -> Specific column, unique to this table

TAXTYPE_2

rndind -> Differently named Id column
rndamb -> Common but differently named column
rndorc -> Specific column, unique to this table

TAXTYPE_3

dasfnd -> Differently named Id column
dastmb -> Common but differently named column
dascrc -> Specific column, unique to this table

(Yes my columns are named like that. Old system. Can't change it. Kill me now, please)

EDIT: I've made the schema clearer. Also, I want to make my point somewhat clearer: the three tables are of the same type, and I'd like to be able to abstract a supertype implementing the common fields and then write the specifics into each subclass. So I'd have a TaxType class and a TaxType1, TaxType2 and TaxType3 subclasses, and each one of them "is a" TaxType. Also, this would make my other layers easier to use since I'd do queries with a single TaxType repository and so on. Thanks to Jamie Ide for making me realize how badly was the question asked.

just to be sure: are the ID's unique among tables? how they are generated right now? — Radim Köhler, Dec 07 '12 at 13:56
They're autosequencial integers, unique on its own table. I mean, there's a TAXTYPE_1.clcpnd = 1 and a TAXTYPE_2.rndind = 1 and a TAXTYPE_3.dasfnd = 1. — CMPerez, Dec 08 '12 at 10:22
In this case, when the same `ID` value could belong to different *subtypes*, you cannot profit from NHibernate inheritance functionality. There simply won't be unique identifier when working with *superclass* `TaxType` to distinguish instances. — Radim Köhler, Dec 08 '12 at 10:58

score 1 · Answer 1 · answered Dec 07 '12 at 12:59

It's a common mistake to use inheritance because your objects share properties. Inheritance is appropriate when your objects have shared behavior and/or have an "is-a" relationship (e.g. car is a vehicle). Also consider how you will query for the objects; do you need to query for a base class and have NHibernate return any derived class?

An interface may be a better choice in your case. This also resolves the differently named Id columns because you can define a property named Id that accesses the actual column.

public interface IMyEntity
{
    int Id { get; set; }
}

public class MyClass : IMyEntity
{
    public int Clcnmb { get; set; }
    public int Id
    { 
        get { return Clcnmb; }
        set { Clcnmb = value; }
    }
}

I'm gonna update the question with additional information, since you've made obvious that my question dfoes not state clearly my intentions and my problem. — CMPerez, Dec 07 '12 at 13:13

score 1 · Accepted Answer · answered Dec 08 '12 at 16:10

As discussed in the comments: The problem here is the existing ID generation strategy. While one value of the ID (e.g. 1) can be used in all tables, NHibernate mechanism for inheritance can not be used.

The base, abstract class TaxType requires unique ID's among its subtypes, to distinguish each instance.

Inheritance mapping in NHibernate with different named Id columns

2 Answers2