Ну, я собирался просто удалить свой вопрос, так как я разобрался с ним, но, поскольку голосование идет вверх, я опубликую найденное мной решение
List<CategoryModel> list =
(from category in context.GetEvents(date, date) // Stored Procedure
group category by category.CategoryId
into groupedByCategory // Group by Category
select new CategoryModel
{
CategoryId = groupedByCategory.Key,
Events = new ObservableCollection<EventModel>
(
from evt in groupedByCategory
group evt by new { evt.EventId, evt.AssignedUserId, evt.EventName, evt.EventDescription, evt.IsRecurring }
into groupedByEvent // Sub-Group for Events
select new EventModel
{
EventId = groupedByCategory.Key,
AssignedUserId = groupedByEvent.Key.AssignedUserId,
EventDescription = groupedByEvent.Key.EventDescription,
EventName = groupedByEvent.Key.EventName,
IsRecurring = groupedByEvent.Key.IsRecurring,
Instances = new ObservableCollection<EventInstanceModel>
(
from item in groupedByEvent
select new EventInstanceModel
{
CompletedDate = item.CompletedDate,
CompletedUserId = item.CompletedUserId,
DueDate = item.DueDate,
EventInstanceId = item.EventInstanceId,
IsSkipped = item.IsSkipped.GetValueOrDefault()
}
)
}
)
}
).ToList();