How to get arrays on output from MongoDB aggregate

Question

I have documents in MongoDB, like so:

{
    "_id" : ObjectId("5a748c8b178227d602ec9ce8"),
    "dateHour" : ISODate("2018-02-02T16:00:00.000Z"),
    "data" : [ 
        {
            "date" : ISODate("2018-02-02T16:06:35.033Z"),
            "cap" : 437105726836.0
        }, 
        {
            "date" : ISODate("2018-02-02T16:09:25.127Z"),
            "cap" : 437316498502.0
        },
        ...
    ]
}

Using aggregate method (in NodeJS):

db.getCollection('hourly').aggregate([
    {$match: {}}, 
    {$unwind: "$data"}, 
    {$project: {_id: 0, date: "$data.date", cap: "$data.cap" } }
])

I get output like:

[
    {
        "date" : ISODate("2018-02-02T16:06:35.033Z"),
        "cap" : 437105726836.0
    },
    {
        "date" : ISODate("2018-02-02T16:09:25.127Z"),
        "cap" : 437316498502.0
    }
]

QUESTION: What is the most effective way to get output like so:

[
    [ISODate("2018-02-02T16:06:35.033Z"), 437105726836.0],
    [ISODate("2018-02-02T16:09:25.127Z"), 437316498502.0]
]

?

I can simply add .map(function(item) {return [item.date, item.cap]}) but is this most effective way when working with huge amount of data?

Saravana · Accepted Answer · 2018-02-03 04:24:56Z

2

try $project with $map or $reduce

$map

db.col.aggregate(
    [
        {$project : {
            _id : 0, 
            data : {$map : {input : "$data", as : "d", in : ["$$d.date", "$$d.cap"]}}
            }
        }
    ]
)

$reduce

db.col.aggregate(
    [
        {$project : {
            _id : 0, 
                data : {$reduce : {input : "$data", initialValue : [], in : {$concatArrays : ["$$value", [["$$this.date", "$$this.cap"]]]}}}
            }
        }
    ]
).pretty()

output

{
    "data" : [
        [
            ISODate("2018-02-02T16:06:35.033Z"),
            437105726836
        ],
        [
            ISODate("2018-02-02T16:09:25.127Z"),
            437316498502
        ]
    ]
}

edited Feb 3, 2018 at 4:24

answered Feb 3, 2018 at 4:02

Saravana

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Comments

Kevin Smith · Accepted Answer · 2018-02-02 19:41:53Z

The root has to be a document, proof:

db.test.aggregate([
    {$unwind: "$data"}, 
    { $replaceRoot: { newRoot: ["$data.date", "$data.cap"] } }
]);
assert: command failed: {
        "ok" : 0,
        "errmsg" : "'newRoot' expression must evaluate to an object, but resulting value was: [null, null]. Type of resulting value: 'array'. Input document: {date: 2018-02-02T16:06:35.033Z, cap: 4.37106e+11}",
        "code" : 40228,
        "codeName" : "Location40228"
} : aggregate failed

You could, however, project it into an array within a document:

> db.test.aggregate([
...     {$unwind: "$data"},
... { $replaceRoot: { newRoot: {a:["$data.date", "$data.cap"] } }}
... ])
{ "a" : [ ISODate("2018-02-02T16:06:35.033Z"), 437105726836 ] }
{ "a" : [ ISODate("2018-02-02T16:09:25.127Z"), 437316498502 ] }

Jeff Matthews · Accepted Answer · 2018-02-02 21:31:10Z

0

It's in the projection. Try this:

db.getCollection('hourly').aggregate([
   {$match: {}}, 
   {$unwind: "$data"}, 
   {$project: {_id: 0, date: ["$data.date", "$data.cap"] } }
]);

Just in case my syntax is a little off, here is MongoDb documentation to project a new array.

I don't see why you need aggregate.

Why not:

db.getCollection('hourly').find({}, {data: 1}, (err, results) => {
   // manage results here.
});

edited Feb 2, 2018 at 21:31

answered Feb 2, 2018 at 21:16

Jeff Matthews

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3 Comments

Sergio Over a year ago

On your first aggregate, result like so:

[     {date: [ISODate("2018-02-02T16:06:35.033Z"), 437105726836.0]},     {date: [ISODate("2018-02-02T16:09:25.127Z"), 437316498502.0]} ]

Sergio Over a year ago

On second request result also not useful, like so:

{     "data" : [          {             "date" : ISODate("2018-02-02T16:06:35.033Z"),             "cap" : 437105726836.0,         },          {             "date" : ISODate("2018-02-02T16:09:25.127Z"),             "cap" : 437316498502.0,         },         ... }, {     "data" : [          {             "date" : ISODate("2018-02-02T16:06:35.033Z"),             "cap" : 437105726836.0,         },          ...

Jeff Matthews Over a year ago

On the first one, what's the problem? It has your fields in an array of arrays, which looks like what you are seeking. On the second one, it looks like you did not paste all of it. So, why can't you use this format? Maybe you can edit your question and explain.

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