MongoDB aggregate fields without knowing all the fields before hand

Question

How can I compute the aggregate of the following metrics without knowing all the metrics before hand? Can I accomplish this using the aggregate framework or MapReduce?

Answer 1

You can try below aggregation.

Convert the object into array of key value pairs followed by $unwind+$group to group by each key and accumulate the count. Final step to go back to named key value object.

db.colname.aggregate([
  {"$addFields":{"metrics":{"$objectToArray":"$metrics"}}},
  {"$unwind":"$metrics"},
  {"$group":{
    "_id":{"id":"$player_id","key":"$metrics.k"},
    "count":{"$sum":"$metrics.v"}
  }},
  {"$group":{
    "_id":"$_id.id",
    "metrics":{"$mergeObjects":{"$arrayToObject":[[["$_id.key","$count"]]]}}
  }}
])