Question

Assuming that writing nhibernate mapping files is not a big issue….or polluting your domain objects with attributes is not a big issue either….

what are the pros and cons?

is there any fundamental technical issues? What tends to influence peoples choice?

not quite sure what all the tradeoffs are.

Answer 1

The biggest pro of AR is that it gives you a ready-made repository and takes care of session management for you. Either of ActiveRecordBase<T> and ActiveRecordMediator<T> are a gift that you would have ended up assembling yourself under NHibernate. Avoiding the XML mapping is another plus. The AR mapping attributes are simple to use, yet flexible enough to map even fairly ‘legacy’ databases.

The biggest con of AR is that it actively encourages you to think incorrectly about NHibernate. That is, because the default session management is session-per-call, you get used to the idea that persisted objects are disconnected and have to be Save()d when changes happen. This is not how NHibernate is supposed to work – normally you’d have session-per-unit-of-work or request or thread, and objects would remain connected for the lifecycle of the session, so changes get persisted automatically. If you start off using AR and then figure out you need to switch to session-per-request to make lazy loading work – which is not well explained in the docs – you’ll get a nasty surprise when an object you weren’t expecting to get saved does when the session flushes.

Bear in mind that the Castle team wrote AR as a complementary product for Castle Monorail, which is a Rails-like framework for .NET. It was designed with this sort of use in mind. It doesn’t adapt well to a more layered, decoupled design.

Use it for what it is, but don’t think about it as a shortcut to NHibernate. If you want to use NH but avoid mapping files, use NHibernate Attributes or better, Fluent NHibernate.