MongoDB Aggregate Lookup and Unwind [duplicate]

Here is one solution - not sure just now if it's the prettiest possible but it certainly gets the job done:

db.transfers.aggregate([{    $lookup: {        from: 'services',        localField: 'requests.service',        foreignField: '_id',        as: 'requests2'    }}, {    $project: {        "requests": {            $map: {                "input": {                    $zip: {                        "inputs": [ "$requests", "$requests2" ]                    }                },                "as": "this",                "in": {                    $mergeObjects: [                        { $arrayElemAt: [ "$$this", 0 ] },                        { "service": { $arrayElemAt: [ "$$this", 1 ] } }                    ]                }            }        }    }}])

The second option is to do this:

db.transfers.aggregate([{    $lookup: {        from: 'services',        localField: 'requests.service',        foreignField: '_id',        as: 'requests2'    }}, {    $project: {        "requests": {            $map: {                "input": {                    $range: [ 0, { $size: "$requests2" } ]                },                "as": "index",                "in": {                    $mergeObjects: [                        { $arrayElemAt: [ "$requests", "$$index" ] },                        { "service": { $arrayElemAt: [ "$requests2", "$$index" ] } }                    ]                }            }        }    }}])

I've never actually compared the performance of the two verions but I would suspect the second is a little bit faster.