您需要先使用
Include(),然后从结果查询中检索单个对象:
Item item = db.Items .Include(i => i.Category) .Include(i => i.Brand) .SingleOrDefault(x => x.ItemId == id);

您需要先使用
Include(),然后从结果查询中检索单个对象:
Item item = db.Items .Include(i => i.Category) .Include(i => i.Brand) .SingleOrDefault(x => x.ItemId == id);