property is filled, but empty in a method (linq select clause)

Question

I have the following code:

private IEnumerable<TrainScheduleViewModel> ListSelect(IQueryable<TrainSchedule> query, Page page)
{
    List<TrainScheduleViewModel> viewModels = query
        .Select(t => new TrainScheduleViewModel
            {
                Id = t.Id,
                ...
                DaysOfWeek = BuildDaysOfWeek(t.DaysOfWeek)
            })
        .Page(page)
        .ToList();

    return viewModels;
}

private string BuildDaysOfWeek(IEnumerable<DayOfWeek> daysOfWeek)
{
    var daysOfWeekStr = string.Empty;

    return daysOfWeek.Aggregate(daysOfWeekStr,
        (current, dayOfWeek) => current + ";" + CultureInfo.CurrentCulture.DateTimeFormat.DayNames[(int) dayOfWeek]);
}

In the debuger the query have one object and DaysOfWeek property is filled. But, in the BuildDaysOfWeek method the collection is empty. Can you explain me why?

Two images:

PS.1 This is mapping for property:

<bag name="DaysOfWeek" cascade="all-delete-orphan" table="`ScheduleOnDaysOfWeek`">
      <key column="`trainScheduleId`" not-null="true"/>
      <element  column="`dayOfWeek`" type="System.DayOfWeek" not-null="true" />
    </bag>

Answer 1

If DaysOfWeek a child collection? If so you need to eager load that table in order to do that type of projection e.g.

List<TrainScheduleViewModel> viewModels = query
    .Select(t => new TrainScheduleViewModel
                {
                    Id = t.Id,
                    ...
                    DaysOfWeek = BuildDaysOfWeek(t.DaysOfWeek)
                })
            .Page(page)
            .Fetch(x => x.DaysOfWeek)
            .ToList();

Or alternatively

DaysOfWeek = BuildDaysOfWeek(t.DaysOfWeek.ToList())