mongoDB聚合：如何使用值作为键

Question

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}}

Answer 1

您可以使用这个$group $arrayToObject

db.col.aggregate([
   {"$group" : {"_id" : "$user","data" : {"$push" : {"k" : "$item","v" : "$count"}}}},
   { "$project" : {"user" : "$_id","_id" : 0, "data" : { "$arrayToObject" : "$data" }}}

])