NHibernate: One base class, several mappings

Question

I'm relatively new to NHibernate, but have been using it for the last few programs and I'm in love. I've come to a situation where I need to aggregate data from 4-5 databases into a single database. Specifically it is serial number data. Each database will have its own mapping file, but ultimately the entities all share the same basic structure (Serial class).

I understand NHibernate wants a mapping per class, and so my initial thought was to have a base Serial Class and then inherit from it for each different database and create a unique mapping file (the inherited class would have zero content). This should work great for grabbing all the data and populating the objects. What I would then like to do is save these inherited classes (not sure what the proper term is) to the base class table using the base class mapping.

The problem is I have no idea how to force NHIbernate to use a specific mapping file for an object. Casting the inherited class to the base class does nothing when using 'session.save()' (it complains of no mapping).

Is there a way to explicitly specify which mapping to use? Or is there just some OOP principal I am missing to more specifically cast an inherited class to base class? Or is this idea just a bad one.

All of the inheritance stuff I could find with regards to NHibernate (Chapter 8) doesn't seem to be totally applicable to this function, but I could be wrong (the table-per-concrete-class looks maybe useful, but I can't wrap my head around it totally with regards to how NHibernate figures out what to do).

Answer 1

I don't know if this'll help, but I wouldn't be trying to do that, basically.

Essentially, I think you're possibly suffering from "golder hammer" syndrome: when you have a REALLY REALLY nice hammer (i.e. Hibernate (and I share your opinion on it; it's a MAGNIFICENT tool)), everything looks like a nail.

I'd generally try to simply have a "manual conversion" class, i.e. one which has constructors which take the hibernate classes for your individual Serial Classes and which simply copies the data over to its own specific format; then Hibernate can simply serialize it to the (single) database using its own mapping.

Effectively, the reason why I think this is a better solution is that what you're effectively trying to do is have asymmetric serialization in your class; i.e. read from one database in your derived class, write to another database in your base class. Nothing too horrible about that, really, except that it's fundamentally a unidirectional process; if you really want conversion from one database to the other, simply do the conversion, and be over with it.

Answer 2

This might help;

Using NHibernate with Multiple Databases

From the article;

Introduction

... described using NHibernate with ASP.NET; it offered guidelines for communicating with a single database. But it is sometimes necessary to communicate with multiple databases concurrently. For NHibernate to do this, a session factory needs to exist for each database that you will be communicating with. But, as is often the case with multiple databases, some of the databases are rarely used. So it may be a good idea to not create session factories until they're actually needed. This article picks up where the previous NHibernate with ASP.NET article left off and describes the implementation details of this simple-sounding approach. Although the previous article focused on ASP.NET, the below suggestion is supported in both ASP.NET and .NET.

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The first thing to do when working with multiple databases is to configure proper communications. Create a separate config file for each database, put them all into a central config folder, and then reference them from the web/app.config.

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