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

Question

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

{
  _id : ObjectId(\"sdfsdf\"),
  result : [1, 3         


        

Answer 1

You can use includeArrayIndex if you have 3.2 or newer MongoDb.

Then you should change $unwind.

Your code should be like this:

.aggregate(
    [
      {
        "$unwind" :  { path: "$result", includeArrayIndex: "arrayIndex" }
      },
      {
        "$group": {
          "_id": "$arrayIndex",
          "results" : { "$sum" : "$result"}
          }
      },
      { 
        $sort: { "_id": 1}
      },
      {
        "$group":{
          "_id": null,
          "results":{"$push":"$results"}
          } 
      },
      {
        "$project": {"_id":0,"results":1}
      }
    ]
)