A test for RavenDB where DocumentQuery doesn't support OfType, although the LINQ Query does support it.

DocumentQueryTests.cs

public class DocumentQueryTests : RavenTestBase
{
    [Fact]
    public void CanUseOfTypeToChangeTheType()
    {
        using (var store = NewDocumentStore())
        {
            new Employee_ForSearch().Execute(store);

            using (var session = store.OpenSession())
            {
                session.Store(new Employee
                {
                    FirstName = "Max",
                    LastName = "Muster",
                });

                session.Store(new Employee
                {
                    LastName = "Apple",
                    FirstName = "Cola"
                });

                session.Store(new Employee
                {
                    LastName = "Muster",
                    FirstName = "Maxi"
                });

                session.SaveChanges();
            }

            WaitForIndexing(store);

            using (var session = store.OpenSession())
            {
                //This works, no issue here
                var employees = session.Query<Employee_ForSearch.Result, Employee_ForSearch>()
                    .Search(f => f.Query, "max")
                    .OfType<Employee>()
                    .ToList();

                Assert.NotEmpty(employees);
                Assert.Equal(employees.Count, 1);
            }

            using (var session = store.OpenSession())
            {
                //Here it fails
                var employees = session.Advanced.DocumentQuery<Employee_ForSearch.Result, Employee_ForSearch>()
                    .Search(f => f.Query, "max")
                    .Cast<Employee>()
                    .ToList();

                Assert.NotEmpty(employees);
                Assert.Equal(employees.Count, 1);
            }
        }
    }
}

public class Employee_ForSearch : AbstractIndexCreationTask<Employee, Employee_ForSearch.Result>
{
    public class Result
    {
        public string[] Query { get; set; }
    }

    public Employee_ForSearch()
    {
        this.Map = employees =>
            from employee in employees
            select new Result
            {
                Query = new[] {employee.FirstName, employee.LastName}
            };

        this.Index(f => f.Query, FieldIndexing.Analyzed);
    }
}

public class Employee
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}