Actually, there are 2 tests/bugs here. First for subj, second for new session modes.

namespace Project2
{
    using System.Linq;
    using System.Linq.Expressions;
    using System.Reflection;
    using System.Transactions;

    using NUnit.Framework;

    using Xtensive.Core;
    using Xtensive.Linq;
    using Xtensive.Orm.Configuration;

    using System;

    using Xtensive.Orm;

    [Serializable]
    [HierarchyRoot]
    public class Person : Entity, ITest
    {
        [Field, Key]
        public int Id { get; private set; }

        [Field]
        public int Age { get; set; }

        [Field]
        public Region Region { get; set; }

        public string RegionName { get { return exprComp(Region); } }

        private static readonly Expression<Func<Region, string>> expr = r => r.Name;

        private static readonly Func<Region, string> exprComp = expr.Compile();

        /// <summary>
        /// The custom linq compiler container.
        /// </summary>
        [CompilerContainer(typeof(Expression))]
        public static class CustomLinqCompilerContainer
        {
            /// <summary>
            /// The current.
            /// </summary>
            /// <param name="assignmentExpression">
            /// The assignment expression.
            /// </param>
            /// <returns>
            /// </returns>
            [Compiler(typeof(Person), "RegionName", TargetKind.PropertyGet)]
            public static Expression Current(Expression assignmentExpression)
            {
                return expr.BindParameters(assignmentExpression);
            }
        }
    }

    [Serializable]
    [HierarchyRoot]
    public class Region : Entity
    {
        [Field, Key]
        public int Id { get; private set; }

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

    public interface ITest : IEntity
    {
        string RegionName { get; }
    }

    /// <summary>
    /// The virtual fields test.
    /// </summary>
    [TestFixture]
    public class VirtualFieldsTest
    {
        private Domain domain;

        [TestFixtureSetUp]
        public virtual void Setup()
        {
            var cfg = new DomainConfiguration("sqlserver", "Data Source=.; Initial Catalog=DO40-Tests; Integrated Security=True;Connection Timeout=300")
            {
                UpgradeMode = DomainUpgradeMode.Recreate,
                ValidationMode = ValidationMode.OnDemand,
                NamingConvention = new NamingConvention { NamingRules = NamingRules.UnderscoreDots }
            };

            cfg.Sessions.Add(new SessionConfiguration("Default")
            {
                BatchSize = 25,
                DefaultIsolationLevel = IsolationLevel.ReadCommitted,
                CacheSize = 1000,
                Options = SessionOptions.Default | SessionOptions.AutoTransactionOpenMode | SessionOptions.AutoActivation
            });

            cfg.Types.Register(Assembly.GetExecutingAssembly());

            domain = Domain.Build(cfg);
        }

        /// <summary>
        /// Проверка виртуальных полей
        /// </summary>
        [Test]
        [Transactional(ActivateSession = true)]
        public void VirtualFieldSelect1()
        {
        }

        /// <summary>
        /// Проверка виртуальных полей
        /// </summary>
        [Test]
        public void VirtualFieldSelect2()
        {
            using (var s = domain.OpenSession())
            {
                var r = new Region { Name = "13123123121" };
                var p = new Person { Age = 1, Region = r };

                var queryable = Query.All<Person>();

                var qwe = from person in queryable
                          where person.RegionName != null
                          select person;
                Assert.DoesNotThrow(() => qwe.ToArray());

                var q = queryable as IQueryable<ITest>;
                var qq = from test in q
                         where test.RegionName != null
                         select test;
                Assert.DoesNotThrow(() => qq.ToArray());
            }
        }
    }
}