嗨，我有一些记录，例如：
{_id:xxx, user:1, item:A, count: 1}
{_id:xxx, user:1, item:B, count: 3}
{_id:xxx, user:2, item:A, count: 5}
{_id:xxx, user:2, item:B, count: 2}
我可以使用聚合得到下面的字典吗？
{user:1,{A:1,B:3}, user:2,{A:5,B:2}}