Fluent NHibernate and computed properties

I'm using Fluent NHibernate, and auto-mapping the classes.

I have a computed property in a class along the lines of

public virtual DateTime? LastActionTimeStamp
{
    get {
        return Actions.Count == 0 ? null : Actions.OrderByDescending(
            a => a.TimeStamp).ElementAt(0).TimeStamp;
    }
}

This wasn't mapped with the rest of the properties, so I couldn't use it in an ICriteria restriction. I added an empty setter (as I read somewhere that doing so would include it in the mapping, which it does), but now I'm getting an NHibernate error:

could not execute query ... Invalid column name 'LastActionTimeStamp'.

So my question is: how do I tell Fluent NHibernate to tell NHibernate to ignore the database for this property, but still return the calculated value from the property get?

You could associate a formula with the property and use that instead of the c# code. e.g.

Property:

private int _postCount = 0;
public virtual int PostCount
{
    get
    {
        return _postCount;
    }
}

Formula:

(SELECT count(p.ID) FROM BlogPost p WHERE p.BlogID = ID and p.IsDeleted = 0)

And then you can use PostCount in your expression as usual. Remember to set the access modifier on the property in your FluentMapping. Not sure what Fluent supports but the options I have found for normal mapping are:

* property
* field
* field.camelcase
* field.camelcase-underscore
* field.pascalcase-m-underscore
* field.lowercase-underscore
* nosetter.camelcase
* nosetter.camelcase-underscore
* nosetter.pascalcase-m-underscore
* nosetter.lowercase-underscore

Probably best check out NHibernate documentation for the official list Access Strategies so you combine the access strategy with the naming strategy e.g. "field.lowercase-underscore"