I have following code:

    //AutoMapConfig.cs
using System;
using FluentNHibernate.Automapping;

namespace SimpleFNH.AutoMap
{
    public class AutoMapConfig : DefaultAutomappingConfiguration
    {
        public override bool ShouldMap(Type type)
        {
            return type.Namespace == "Examples.FirstAutomappedProject.Entities";
        }
    }
}
//CascadeConvention.cs
using FluentNHibernate.Conventions;
using FluentNHibernate.Conventions.Instances;

namespace SimpleFNH.AutoMap
{
    public class CascadeConvention : IReferenceConvention, IHasManyConvention, IHasManyToManyConvention
    {
        public void Apply(IManyToOneInstance instance)
        {
            instance.Cascade.All();
        }

        public void Apply(IOneToManyCollectionInstance instance)
        {
            instance.Cascade.All();
        }

        public void Apply(IManyToManyCollectionInstance instance)
        {
            instance.Cascade.All();
        }
    }
}
//Item.cs
namespace SimpleFNH.Entities
{
    public class Item
    {
        public virtual long ID { get; set; }
        public virtual string ItemName { get; set; }
        public virtual string Description { get; set; }
        public virtual OrderItem OrderItem { get; set; }
    }
}
//OrderItem.cs

namespace SimpleFNH.Entities
{
    public class OrderItem
    {
        public virtual long ID { get; set; }
        public virtual int Quantity { get; set; }
        public virtual Item Item { get; set; }
        public virtual ProductOrder ProductOrder { get; set; }

        public virtual void AddItem(Item item)
        {
            item.OrderItem = this;
        }
    }

}

using System;
using System.Collections.Generic;

//ProductOrder.cs
namespace SimpleFNH.Entities
{
    public class ProductOrder
    {
        public virtual long ID { get; set; }
        public virtual DateTime OrderDate { get; set; }
        public virtual string CustomerName { get; set; }
        public virtual IList<OrderItem> OrderItems { get; set; }

        public ProductOrder()
        {
            OrderItems = new List<OrderItem>();
        }

        public virtual void AddOrderItems(params OrderItem[] items)
        {
            foreach (var item in items)
            {
                OrderItems.Add(item);
                item.ProductOrder = this;
            }
        }


    }
}

//NHibernateRepo.cs
using FluentNHibernate.Cfg;
using FluentNHibernate.Cfg.Db;
using NHibernate;
using NHibernate.Criterion;
using NHibernate.Tool.hbm2ddl;

namespace SimpleFNH.Repository
{
    public class NHibernateRepo
    {
        private static ISessionFactory _sessionFactory;
        private static ISessionFactory SessionFactory
        {
            get
            {
                if (_sessionFactory == null)
                    InitializeSessionFactory();
                return _sessionFactory;
            }
        }

        private static void InitializeSessionFactory()
        {
            _sessionFactory =
                Fluently.Configure().Database(
                    MsSqlConfiguration.MsSql2008.ConnectionString(
                        @"server=Amit-PC\SQLEXPRESS;database=SimpleFNH;Trusted_Connection=True;").ShowSql()).
                    Mappings(m => m.FluentMappings.AddFromAssemblyOf<Order>()).ExposeConfiguration(
                        cfg => new SchemaExport(cfg).Create(true, true)).BuildSessionFactory();
        }

        public static ISession OpenSession()
        {
            return SessionFactory.OpenSession();
        }
    }
}
//Program.cs
using System;
using System.Collections.Generic;
using System.Linq;
using SimpleFNH.Entities;
using SimpleFNH.Repository;

namespace SimpleFNH
{
    class Program
    {
        static void Main(string[] args)
        {

            using (var session = NHibernateRepo.OpenSession())
            {
                using (var transaction = session.BeginTransaction())
                {
                    var item1 = new Item { ItemName = "item 1", Description = "test 1" };
                    var item2 = new Item { ItemName = "item 2", Description = "test 2" };
                    var item3 = new Item { ItemName = "item 3", Description = "test 3" };

                    var orderItem1 = new OrderItem { Item = item1, Quantity = 2 };
                    var orderItem2 = new OrderItem { Item = item2, Quantity = 4 };
                    var orderItem3 = new OrderItem { Item = item3, Quantity = 5 };

                    var productOrder = new ProductOrder
                                           {
                                               CustomerName = "Amit",
                                               OrderDate = DateTime.Now,
                                               OrderItems = new List<OrderItem> { orderItem1, orderItem2, orderItem3 }
                                           };

                    productOrder.AddOrderItems(orderItem1, orderItem2, orderItem3);

                    session.Save(productOrder);
                    transaction.Commit();
                }
            }

            using (var session = NHibernateRepo.OpenSession())
            {
                // retreive all stores and display them
                using (session.BeginTransaction())
                {
                    var orders = session.CreateCriteria(typeof(ProductOrder))
                        .List<ProductOrder>();

                    foreach (var item in orders)
                    {
                        Console.WriteLine(item.OrderItems.First().Quantity);
                    }
                }
            }
        }
    }
}

I tried many variations to get it working but i get an error saying No persister for: SimpleFNH.Entities.ProductOrder

Can someone help me get it working? I wanted to create a simple program which will set a pattern for my bigger project but it is taking quite a lot of time than expected. It would be rally helpful if you can explain in simple terms on any template/pattern that i can use to get fluent nHibernate working.

The above code uses auto mapping, which i tried after i tried with fluent mapping.