Can the MongoDB aggregation framework $group return an array of values?

Question

How flexible is the aggregate function for output formatting in MongoDB?

Data format:

{
        \"_id\" : ObjectId(\"506ffffd1900a47d802702a904\"),
            


        

Answer 1

MongoDB 2.6 made this a lot easier by introducing $map, which allows a simplier form of array transposition:

db.metrics.aggregate([
   { "$match": {
       "array_serial": array, 
       "port_name": { "$in": ports},
       "datetime": { "$gte": from, "$lte": to }
    }},
    { "$group": {
        "_id": "$port_name",
        "data": {
            "$push": {
                "$map": {
                    "input": [0,1],
                    "as": "index",
                    "in": {
                        "$cond": [
                            { "$eq": [ "$$index", 0 ] },
                            "$datetime",
                            "$metric"
                        ]
                    }
                }
            }
        },
        "count": { "$sum": 1 }
    }}
])

Where much like the approach with $unwind, you supply an array as "input" to the map operation consisting of two values and then essentially replace those values with the field values you want via the $cond operation.

This actually removes all the pipeline juggling required to transform the document as was required in previous releases and just leaves the actual aggregation to the job at hand, which is basically accumulating per "port_name" value, and the transformation to array is no longer a problem area.