I just recently started with NHibernate and have been able to find most of my answers in the forums, documention, and google.

However, I have discovered an issue that someone here might be able to help me with.

Say I have two tables in a database, Device and CalibrationEvent. The calibration event has an associated Device and a time of calibration (CalibrationDate) property.

In my Device mapping file I have CalibrationEvent as a mapped set called PastCalibrations set to lazy load. Now, there can be very large number of CalibrationEvents for a Device. I'm trying to figure out a way to create a property for the Device object that returns the most recent calibration date without iterating through the entire set PastCalibrations.

In my application, I'd like to say something like "myDevice.MostRecentCalibration.CalibrationDate" and get the information I'm looking for. Should I be tyring to do this another way?

I was thinking of having the mapping order the set decending by CalibrationDate and then have the MostRecentCalibration property return the first element in the set. Will this cause the entire PastCalibrations set to be lazy-loaded from the database or just the first element?

I would appreciate any tips or suggestions....even "RTFM" as I'd be happy to know it is in there and I just need to read it more closely.

I believe this is what Filter are for. I haven't tryied them yet since my current project has not gone that far (yet), but basically you do something like this



I'm not sure this code is even right so let us know your results.

I haven't been using NHibernate long so I could be completely off base here, but you could try looking at using the "where" attribute on the "set" element. You could map the set using the "field" access strategy and keep the field private. And then in your MostRecentCalibration getter, get the first element of the private Set field.

roniburd,

I think filters would work great, but which layer do they belong in? If I put it in my Device class, doesn't that tie the domain layer to the data layer? I've never found an example of anyone putting nhibernate stuff in with their domain classes.

I guess if I explain my model a little better.....
I have web, data, service, and domain layers. The web layer calls static methods of the service classes (like DeviceService.GetAllDevices()), which in turn use a session (created at beginning of request and flushed at end) managed by the data layer and return collections of domain objects from the domain layer.

Can you recommend an example project that has good design that I can look at? I've been looking at the Cuyahoga project (http://www.martijnboland.com/cuyahoga/).

Thanks for all your help so far.

I ended up creating a property of the Device class that used a sql formula.

Here is the interesting piece of my xml file for Device:


Does this seem reasonable?