Null values from string[].FirstOrDefault() can't be queried

[Andrej Krivulčík]

When using map-reduce index and there is a field retrieved using FirstOrDefault(), which returns (string)null, the records can't be queried using [[NULL_VALUE]].

When looking at Terms in Studio, the NULL_VALUE is not present.

Failing test:

    public class StringIndexingTest : RavenTestBase
    {
        public class Employee
        {
            public string Id { get; set; }
            public string FirstName { get; set; }
            public string StrVal { get; set; }
        }

        public class EmployeeView
        {
            public string Id { get; set; }
            public string FirstName { get; set; }
            public string StrVal { get; set; }
        }

        public class EmployeeIndex : AbstractMultiMapIndexCreationTask<EmployeeView>
        {
            public EmployeeIndex()
            {
                AddMap<Employee>(
                    employees => from employee in employees
                                 select new
                                 {
                                     employee.Id,
                                     employee.FirstName,
                                     employee.StrVal,
                                 });

                Reduce = results => from result in results
                                    group result by result.Id
                                    into g
                                    let employee = g.First()
                                    select new
                                    {
                                        Id = employee.Id,
                                        FirstName = employee.FirstName,
                                        StrVal = g.Select(x => x.StrVal).FirstOrDefault(x => !string.IsNullOrEmpty(x)), // results of this construct can't be queried using [[NULL_VALUE]]
                                    };
            }
        }

        [Fact]
        public void StringArraysWithNoDataCanBeQueried()
        {
            using (var store = NewRemoteDocumentStore())
            {
                new EmployeeIndex().Execute(store);

                using (var session = store.OpenSession())
                {
                    session.Store(new Employee{ Id = "1", FirstName = "A", StrVal = null });
                    session.Store(new Employee{ Id = "2", FirstName = "B", StrVal = "B" });
                    session.SaveChanges();
                }

                using (var session = store.OpenSession())
                {
                    session.Query<EmployeeView, EmployeeIndex>()
                        .Customize(x => x.WaitForNonStaleResults())
                        .Count();
                }

                using (var session = store.OpenSession())
                {
                    var nonNullCount = session.Query<EmployeeView, EmployeeIndex>()
                        .Where(x => x.StrVal != null)
                        .Count();
                    Assert.Equal(1, nonNullCount); // works correctly

                    var nullCount = session.Query<EmployeeView, EmployeeIndex>()
                        .Where(x => x.StrVal == null)
                        .Count();
                    Assert.Equal(1, nullCount); // doesn't find the document
                }
            }
        }
    }

[Oren Eini (Ayende Rahien)]

Use:

 g.Select(x => x.StrVal).FirstOrDefault(x => !string.IsNullOrEmpty(x)) ?? null;

The reason is that this is effectively an missing value, so we don't index it.

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[Andrej Krivulčík]

Is the entire record not present in the index in this case?

[Oren Eini (Ayende Rahien)]

Yes, but the specific field isn't

[Andrej Krivulčík]

Is there a reason for this handling? What's the difference between null value and missing value in this case? Is there a use case for this where it's desirable to not use null as the result of FirstOrDefault() automatically (without explicitly stating it with ?? null)?

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[Oren Eini (Ayende Rahien)]

The problem is that we are doing null handling, so you can do things like:

                                        StrVal = g.Select(x => x.StrVal).FirstOrDefault(x => !string.IsNullOrEmpty(x)).ToUpper()

And it will work without throwing an NRE.

When we get to indexing, we assume that this is a missing value and not important, so we don't index it.

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[Andrej Krivulčík]

Makes sense, thanks.