I'm just wondering what the best practise for updating a static property of an object is. The thing I'm worried about is concurrency. For example, a Product object might have a quantity property. When this Product is purchased you, would decrement the quantity property by 1. However, if you simply quantity-- and save the object you could have problems. Imagine Product1 was loaded and Product2 was loaded at the same time. The quantity at this point is 35. Product1 is saved so quantity goes down to 34. Now Product2 is saved. If it's quantity is 35 and decremented, then the quantity is 34. In reality the quantity should be 33. Do you just write custom sql to take care of cases like this?

Yes, you are right - it's better to accomplish with custom SQL statement executed in the scope of the same transaction, which inserts/updates other purchase-related records.

Perhaps, you can also start transaction, then load your object with the property you need to update, ensuring that its record is locked in the DB, then change property, save object and commint transaction. Maybe it's achievable by using LockMode.Upgrade, but i'm not sure, because haven't tried.

I would consider handling this either through the NHibernate interceptor or through a database trigger.