change key document mongodb aggregate

To create a nested object with dynamic key you need to use $arrayToObject which takes an array of keys (k) and values (v) as a parameter. Then you can use $replaceRoot to promote that new object to a root level. You need MongoDB 4.0 to convert ObjectId to string using $toString operator

db.col.aggregate([    {        $replaceRoot: {            newRoot: {                $arrayToObject: {                    $let: {                        vars: { data: [ { k: { $toString: "$_id" }, v: "$serie" } ] },                        in: "$$data"                    }                }            }        }    }])

Outputs:

{    "5b97f6cea37f8c96db70fea9" : {            "_id" : ObjectId("5a55f988b6c9dd15b47faa2a"),            "updatedAt" : ISODate("2018-02-09T13:22:54.521Z"),            "createdAt" : ISODate("2018-01-10T11:31:20.978Z"),            "deletar" : false,            "infantil" : false,            "status" : true,            "turma" : [ ]    }}

If you want to get rid of _id in your result you can specify fields explicitly in v like:

v: { updatedAt: "$serie.updatedAt", createdAt: "$serie.createdAt", ... }

You can use below aggregation pipeline in 4.0.

db.colname.aggregate([  {"$addFields":{"idstr":{"$toString":"$_id"}}},  {"$project":{"serie._id":0}},  {"$replaceRoot":{"newRoot":{"$arrayToObject":[[["$idstr","$serie"]]]}}}])

You can use one more trick using $group aggregation stage.

db.collection.aggregate([  { "$group": {    "_id": null,    "data": { "$push": { "k": { "$toString": "$serie._id" }, "v": "$serie" }}  }},  { "$replaceRoot": { "newRoot": { "$arrayToObject": "$data" }}}])