MongoDB sum arrays from multiple documents on a per-element basis

Ask Time：2016-02-18T19:00:00 Author：Philip O'Brien

I have the following document structure (simplified for this example)

{
  _id : ObjectId("sdfsdf"),
  result : [1, 3, 5, 7, 9]
},
{
  _id : ObjectId("asdref"),
  result : [2, 4, 6, 8, 10]
}

I want to get the sum of those result arrays, but not a total sum, instead a new array corresponding to the sum of the original arrays on an element basis, i.e.

result : [3, 7, 11, 15, 19]

I have searched through the myriad questions here and a few come close (e.g. this one, this one, and this one), but I can't quite get there.

I can get the sum of each array fine

aggregate(
    [
      {
        "$unwind" : "$result"
      },
      {
        "$group": {
          "_id": "$_id",
          "results" : { "$sum" : "$result"}
          }
      }
    ]
)

which gives me

[ { _id: sdfsdf, results: 25 },
  { _id: asdref, results: 30 } ]

but I can't figure out how to get the sum of each element

Author:Philip O'Brien，eproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/35479563/mongodb-sum-arrays-from-multiple-documents-on-a-per-element-basis

Erkan Demirel :

You can use includeArrayIndex if you have 3.2 or newer MongoDb.\n\nThen you should change $unwind.\n\nYour code should be like this:\n\n.aggregate(\n [\n {\n \"$unwind\" : { path: \"$result\", includeArrayIndex: \"arrayIndex\" }\n },\n {\n \"$group\": {\n \"_id\": \"$arrayIndex\",\n \"results\" : { \"$sum\" : \"$result\"}\n }\n },\n { \n $sort: { \"_id\": 1}\n },\n {\n \"$group\":{\n \"_id\": null,\n \"results\":{\"$push\":\"$results\"}\n } \n },\n {\n \"$project\": {\"_id\":0,\"results\":1}\n }\n ]\n)\n",

2016-02-18T12:51:17

MongoDB sum arrays from multiple documents on a per-element basis