DO version 5.0.6

Model:

[HierarchyRoot]
[Serializable]
public class TestEntity : Entity
{
    [Key]
    [Field(Nullable = false)]
    public Guid Id { get; private set; }

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

    [Field]
    public decimal Sum { get; set; }
}

Test:

    var dc = new DomainConfiguration("sqlserver://localhost/DO40-Tests");

    dc.Types.Register(typeof(TestEntity));

    dc.UpgradeMode = DomainUpgradeMode.Recreate;
    var sessionConfiguration = new SessionConfiguration(SessionOptions.AutoActivation | SessionOptions.ServerProfile);

    using (var domain = Domain.Build(dc))
    {
        using (var session = domain.OpenSession(sessionConfiguration))
        using (session.Activate())
        using (var t = session.OpenTransaction())
        {
            new TestEntity { TestField = "Test1", Sum = 1};
            new TestEntity { TestField = "Test2", Sum = 1 };
            new TestEntity { TestField = "Test3", Sum = 1 };
            new TestEntity { TestField = "Test3", Sum = 1 };

            t.Complete();
        }

        using (var session = domain.OpenSession(sessionConfiguration))
        using (session.Activate())
        using (var t = session.OpenTransaction())
        {
            Assert.AreEqual(0, Session.Current.Query.All<TestEntity>()
                .Select(e => e.TestField.Contains("3") ? e.Sum : -e.Sum)
                .Sum(e => e)); //OK

            Assert.AreEqual(0, Session.Current.Query.All<TestEntity>()
                .Select(e => e.TestField.Contains("3") ? e.Sum : -e.Sum)
                .Sum()); // NOT OK

            t.Complete();
        }
    }

Exception:

An unhandled exception of type 'Xtensive.Orm.QueryTranslationException' occurred in     Xtensive.Orm.dll

Additional information: Unable to translate 'Query.All().Select(e => e.TestField.Contains("3")

? e.Sum

: -(e.Sum)).Sum()' expression. See inner exception for details.

Unable to translate 'All().Select(e => IIF(e.TestField.Contains(\"3\"), e.Sum, -e.Sum)).Sum()' expression. Aggregates for non primitive types are not supported.

----- Edited 24.07.2015

This work

Session.Current.Query.All<TestEntity>()
                    .ToArray()
                    .Select(e => e.TestField.Contains("3") ? e.Sum : -e.Sum)
                    .Sum();