From http://bugsquash.blogspot.com/2009/05/rails-like-finders-for-nhibernate-with.html

nhibernate_ActiveRecord

using System;
using System.Collections;
using System.Collections.Generic;
using System.Dynamic;
using System.Linq;
using System.Text.RegularExpressions;
using Castle.ActiveRecord;
using Castle.ActiveRecord.Framework.Config;
using log4net.Config;
using NHibernate;
using NHibernate.Criterion;
using NUnit.Framework;

namespace NHDynamicTests {
    [TestFixture]
    public class DynamicTests {
        [Test]
        public void DynamicGetPersonByName() {
            using (ISession s = ActiveRecordMediator.GetSessionFactoryHolder().GetSessionFactory(typeof(object)).OpenSession()) {
                dynamic ds = s.AsDynamic();
                Person person = ds.GetPersonByName("pepe");
            }
        }

        [ActiveRecord]
        public class Person {
            [PrimaryKey]
            public int Id { get; set; }

            [Property]
            public string Name { get; set; }
        }

        [TestFixtureSetUp]
        public void FixtureSetup() {
            BasicConfigurator.Configure();
            var arConfig = new InPlaceConfigurationSource();
            var properties = new Dictionary<string, string> {
                {"connection.driver_class", "NHibernate.Driver.SQLite20Driver"},
                {"dialect", "NHibernate.Dialect.SQLiteDialect"},
                {"connection.provider", "NHibernate.Connection.DriverConnectionProvider"},
                {"connection.connection_string", "Data Source=test.db;Version=3;New=True;"},
            };

            arConfig.Add(typeof(ActiveRecordBase), properties);
            ActiveRecordStarter.ResetInitializationFlag();
            var arTypes = GetType().GetNestedTypes()
                .Where(t => t.GetCustomAttributes(typeof(ActiveRecordAttribute), true).Length > 0)
                .ToArray();
            ActiveRecordStarter.Initialize(arConfig, arTypes);
            ActiveRecordStarter.CreateSchema();
        }
    }

    public static class ISessionExtensions {
        public static dynamic AsDynamic(this ISession session) {
            return new DynamicSession(session);
        }
    }

    public class DynamicSession : DynamicObject {
        private readonly ISession session;

        public DynamicSession(ISession session) {
            this.session = session;
        }

        public override bool TryInvokeMember(InvokeMemberBinder binder, object[] args, out object result) {
            result = null;
            if (!binder.Name.StartsWith("Get"))
                return false;
            var tokens = Regex.Replace(binder.Name, "([A-Z])", " $1").Split(' ').Skip(1).ToArray();
            if (tokens.Length < 4 || args.Length < 1)
                return false; // some parameter is missing
            var typeName = tokens[1];
            var type = session.SessionFactory.GetAllClassMetadata()
                .Cast<DictionaryEntry>()
                .Select(e => e.Key).Cast<Type>()
                .FirstOrDefault(t => t.Name == typeName);
            if (type == null)
                throw new ApplicationException(string.Format("Type '{0}' is not mapped in NHibernate", typeName));
            var fieldName = tokens[3];
            var criteria = DetachedCriteria.For(type).Add(Restrictions.Eq(fieldName, args[0]));
            result = criteria.GetExecutableCriteria(session).UniqueResult();
            return true;
        }
    }
}