Map a property to the latest entry in NHibernate

Question

Let's say my domain looks like this:

• I have an object, Vehicle, that has an OdometerReading property.
• An OdometerReading has the Miles & Date (when it was read).

I need to keep a history of all OdometerReadings for the Vehicle in the database, but don't want the entire odometer history to belong to the Vehicle object. What I would like is for the OdometerReading property map to the most recent OdometerReading entry out of the database.

I thought about mapping the whole collection of OdometerReadings to the Vehicle, and having a dynamic property called CurrentOdometerReading that would order them and return the latest one, but I don't need the whole collection under the Vehicle in my domain, and it seems like I would be getting more data out of the database than I need.

Is that possible with NHibernate? How would I map such a thing?

Answer 1

There are a few ways of doing this depending on what you want your domain model to look like. My preferred choice is to use a custom collection type, for example IOdometerReadingCollection, which you can add extra methods to. In this case it might be something like:

public interface IOdometerReadingCollection : IList<OdometerReading>
{
    OdometerReading Latest { get; }
}

This way you can do:

OdometerReading reading = vehicle.OdometerReadings.Latest;

which I prefer to:

OdometerReading reading = vehicle.LatestOdometerReading;

There's lots of documentation about custom collections, which you can find with a simple google search.

If this approach isn't for you there are other options. You may be able to use a property with a formula (I'm not sure if that works with complex types?), or a regular NHibernate association where you'd have the key of the latest OdometerReading on your Vehicle mapping. As you also mentioned you could just load all the OdometerReadings, which depending on your use case might actually be fine.

I hope this helps, or at least points you in the right direction.

Answer 2

There is a "where" clause that you can put in your collection mapping. Check the reference documentation.

Answer 3

I would map the OdometerReading property as a component, then use a named query to ensure it's populated with the latest reading out of the database. (In this example, you'd have a sql-query with a name of "vehicle" that does the SQL to load the Vehicle columns along with the latest Odometer reading)

<class name="Vehicle">
    <property name="Type" not-null="true"/>
    <component name="OdometerReading">
        <property name="Miles" />
        <property name="Date" />
    </component>
    <loader query-ref="vehicle"/>
</class>