namespace Shop.Domain
{
    public class Order : Entity
    {
        private User user;

        public virtual string Name { get; set; }

        public virtual User User
        {
            get
            {
                return this.user;
            }
            set
            {
                if (value == this.user) return;

                if (this.user != null)
                    this.user.RemoveOrder(this);

                if (value != null)
                    value.AddOrder(this);

                this.user = value;
            }
        }
    }
}

using System;
using System.Collections.Generic;

namespace Shop.Domain
{
    public class User : Entity
    {
        private IList<Order> orders = new List<Order>();

        public virtual string Name { get; set; }

        public virtual IEnumerable<Order> Orders { get { return this.orders; } }

        public virtual void AddOrder(Order order)
        {
            if (order == null) throw new ArgumentNullException("order");

            if (!this.orders.Contains(order))
            {
                this.orders.Add(order);
                order.User = this;
            }
        }

        public virtual void RemoveOrder(Order order)
        {
            if (order == null) throw new ArgumentNullException("order");

            if (this.orders.Contains(order))
            {
                this.orders.Remove(order);
                order.User = null;
            }
        }
    }
}

using FluentNHibernate.Mapping;
using Shop.Domain;

namespace NHInfrastructure.Mappings
{
    class UserMap : ClassMap<User>
    {
        public UserMap()
        {
            Id(x => x.ID).GeneratedBy.HiLo("100");
            Map(x => x.Name);

            HasMany(x => x.Orders)
                .Access.ReadOnlyPropertyThroughLowerCaseField()
                .Cascade.SaveUpdate();
        }
    }
}

using FluentNHibernate.Mapping;
using Shop.Domain;

namespace NHInfrastructure.Mappings
{
    class OrderMap : ClassMap<Order>
    {
        public OrderMap()
        {
            Id(x => x.ID).GeneratedBy.HiLo("100");
            Map(x => x.Name);
            
            References(x => x.User)
                .Not.Nullable()
                .Cascade.SaveUpdate();
        }
    }
}

class Program
    {
        static void Main(string[] args)
        {
            var user = new User { Name = "TestUser" };
            var o = new Order { Name = "TestOrder" };
            o.User = user;
            var userID = 0;

            using (var session = NHManager.GetSession())
            {
                using (var tr = session.BeginTransaction())
                {
                    userID = (int)session.Save(user);
                    tr.Commit();
                }
            }

            var u = NHManager.GetSession().Load<User>(userID);
            // На следующей строке получаем исключение.
            var e = u.Orders.Count() == 1;
        }
    }