对展开的LINQ结果集进行分组

我有一个查询，它会产生一个扁平的结果集，但我实际上并不希望它扁平化，我需要对它进行分组。

var test = (from a in _activityNoteService.GetAll()
    from l in a.ActivityNoteLines
    where l.POItemId == poItem.Id && a.ActivityTypeId == 1
    from s in l.StockCatalogueItem.StockInventoryItems
    where s.OriginalActivityNoteLineID == l.Id
    select new
    {
      sii = s.Id,
      sci = s.StockCatalogueItem,
      po = a.PurchaseOrder
    }).ToList();

sii值是我感兴趣的唯一记录，sci和po值是常见的（即重复的）。我如何重构它，使其返回一个sci、一个po和sii记录列表

var test = (from a in _activityNoteService.GetAll()
    from l in a.ActivityNoteLines
    where l.POItemId == poItem.Id && a.ActivityTypeId == 1
    from s in l.StockCatalogueItem.StockInventoryItems
    where s.OriginalActivityNoteLineID == l.Id
    group s by new { sci = s.StockCatalogueItem, po = a.PurchaseOrder  } into g
    select new
    {
      ListOfSii = g.Select(s => s.Id),
      g.Key.sci,
      g.Key.po
    }).ToList();