Project array of Objects to Key Value

Question

After aggregations i get this result,

{
    \"_id\" : {
        \"date\" : ISODate(\"2017-08-30T00:00:00.000Z\")
    },
    \"aggr\" : [ 
        {
            \         


        

Answer 1

Yes, using $arrayToObject and $map to convert the existing array to a format it accepts:

db.collection.aggregate([
  { "$replaceRoot": {
    "newRoot": {
      "$arrayToObject": {
        "$concatArrays": [
          [{ "k": "date", "v": "$_id.date" }],  
          { "$map": {
            "input": "$aggr",
            "in": { "k": "$$this.gender", "v": "$$this.count" }
          }}
        ]
      }
    }    
  }}
])

Of course if this is actually only on the "tail" of an existing aggregation and you don't have at least MongoDB 3.4.4 where the operator is introduced, then you can simply reshape the result in the client code:

db.collection.aggregate([
  // existing pipeline
]).map(d => 
  Object.assign(
    { date: d._id.date },
    d.aggr.reduce((acc,curr) =>
      Object.assign(acc,{ [curr.gender]: curr.count }),{}
    )
  )
)