Mapping a read-only property with no setter using Fluent NHibernate

Question

I have a domain class that looks like this. I want NHibernate to save the current value of LastUpdate when inserting/updating so that I can use it in queries, but to ignore it when retrieving a Foo from the database and let the object itself recalculate the value when I actually access it.

public class Foo {
    public DateTime LastUpdate {
        get {
            /* Complex logic to determine last update by inspecting History */
            return value;
        }
    }
    public IEnumerable<History> History { get; set; }
    /* etc. */
}

My mapping for Foo looks like this:

public class FooMap : ClassMap<Foo> {
    Map(x => x.LastUpdate)
        .ReadOnly();
    HasMany(x => x.History);
    // etc...
}

I thought that ReadOnly() was what I wanted to accomplish this, but when I try to create a SessionFactory I get the following exception:

Error: FluentNHibernate.Cfg.FluentConfigurationException: An invalid or incomplete configuration was used while creating a SessionFactory. Check PotentialReasons collection, and InnerException for more detail.
---> NHibernate.PropertyNotFoundException: Could not find a setter for property 'LastUpdate' in class 'Foo'.

The property doesn't have a setter because it shouldn't be set, only read from. Is ReadOnly() the correct thing to do here? If not, what?

(NHibernate v3.0b1, Fluent NHibernate v1.1)

Answer 1

ReadOnly instructs Fluent NHibernate to not look for changes on this property, this does not equate to a read-only property in compiler-world. Your property isn't read-only in NHibernate's eyes because you're expecting it to be populated from your database. What you need to do is tell NHibernate that it should access the value of that property through a private field with the same name (lowercased) as the property.

Map(x => x.LastUpdate)
  .Access.Field();

There are several alternatives to using Field, which one you use will depend on how you name your private fields.

Answer 2

As far as NHibernate goes, you can map to a field, i.e member variable so Nhibernate can access the member variable directly. So you can create a member variable like _lastUpdate that can be mapped directly. Nhibernate will now have a variable to use and you can control the value separately in your getter because NHibernate will no longer use the property getter. It will save the value, and retrieve it too, but the retrieved value should not matter because as soon as you access through your getter you can recalculate it. Ditto for private variables with no getters or setters.

In a regular hbm you would just map access=field. Everyone does it. Apparently Fluent is not a simple. I don't use Fluent ...

EDIT ...

find whatever the seemingly always moving target for mapping private backing fields is in your version and use that ...