R – Fluent NHibernate table per subclass inheritance mapping

The way I see it you can't really avoid using Session.Load(id) when using NHibernate. As mentioned in the comments this will NOT hit the database, just create a proxy object with the id.

Some possible options:

1. Inject a second generic repository (ServiceRepository) into the controller. I can't really see an issue with that, but for some reason you want to avoid it. You could add a LoadById method to the generic interface and implement that differently in each implementation for NH and EF (or others). In EF impl that method may work just like GetById, while in NH impl it calls Session.Load
2. Implement a non-generic repository for the AR (aggregate root) which in this case would be Ticket. This could have specific methods to Load a Service as well as Ticket.
3. Implement another abstraction on top of the two repositories and inject that into the controller instead of the two repositories in option 1. This could f.ex be a persistance ignorant UnitOfWork as outlined here: Persistance ignorant UoW, or some kind of Application Service that orchestrates the creation of Tickets, or a TicketFactory.

Of the 3 options, option 1 is probably the simplest, while 3 might provide better abstraction and more maintainability down the road.