Reshape MongoDB aggregation result to field value documents

This pipeline should give you the desired result

db.getCollection('yourColl').aggregate([ {     $project: {        "_id": "$_id",        "aaa": { $avg: ["$aa1","$aa2"] },        "bbb": { $avg: ["$bb1","$bb2"] },        "ccc": { $avg: ["$cc1","$cc2"] }     } },  {     $group: {        "_id": "AvgCalc",        "aaa": { $avg: "$aaa" },        "bbb": { $avg: "$bbb" },        "ccc": { $avg: "$ccc" }     }      }, {    $project: {        "items": [             {name: {$literal: "aaa"}, value: "$aaa"},            {name: {$literal: "bbb"}, value: "$bbb"},            {name: {$literal: "ccc"}, value: "$ccc"}        ]    } }, {     $unwind: {         path: "$items"     } }, {     $project: {         _id: 0,         field: "$items.name",         value: "$items.value"     } }])

For your sample data the previous output was

/* 1 */{    "_id" : "AvgCalc",    "aaa" : 58.25,    "bbb" : 70.0,    "ccc" : 73.5}

and with the new pipeline the output is like this

/* 1 */{    "field" : "aaa",    "value" : 58.25}/* 2 */{    "field" : "bbb",    "value" : 70.0}/* 3 */{    "field" : "ccc",    "value" : 73.5}