MongoDB unwind multiple arrays

Question

I have the following documents:

{
  "dates": [
    1399518702000,
    1399126333000,
    1399209192000,
    1399027545000
  ],
  "dress_number": "4",
  "name": "J. Evans",
  "numbers": [
    "5982",
    "5983",
    "5984",
    "5985"
  ]
}

Is it possible unwind data from multiple arrays and get only paired elements from arrays:

{
    "dates": "1399518702000",
    "numbers": "5982"
},
{
    "dates": "1399126333000",
    "numbers": "5983"
},
{
    "dates": "1399209192000",
    "numbers": "5984"
},
{
    "dates": "1399027545000",
    "numbers": "5985"
}

what happen if dates array length and numbers array length not same? — Neo-coder
– Neo-coder, Commented Sep 7, 2016 at 15:11
The dates array element is a string "[1399518702000, 1399126333000, 1399209192000, 1399027545000]", not an array. — chridam
– chridam, Commented Sep 7, 2016 at 15:16

TomG · Accepted Answer · 2016-09-08 11:04:21Z

33

From version 3.2 you can do it with $unwind on both of the arrays, $cmp the indexes, and $match only the equal indexes.

This solution will populate what you wrote in case you have only the example document. If you have more documents I don't know what you expect to get in the output, but it's solvable by grouping by _id of the document.

db.test.aggregate([
    {
        $unwind: {
            path: '$dates',
            includeArrayIndex: 'dates_index',
        }
    },
    {
        $unwind: {
            path: '$numbers',
            includeArrayIndex: 'numbers_index',
        }
    },
    {
        $project: {
            dates: 1,
            numbers: 1,
            compare: {
                $cmp: ['$dates_index', '$numbers_index']
            }
        }
    },
    {
        $match: {
            compare: 0
        }
    },
    {
        $project: {
            _id: 0,
            dates: 1,
            numbers: 1
        }
    }
])

edited Sep 8, 2016 at 11:04

answered Sep 8, 2016 at 10:47

TomG

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3 Comments

corry Over a year ago

If I understood well, it's possible to unwind a maximum of two arrays?

TomG Over a year ago

@corry how did you understand that? you can unwind as many as you want. The problem is that each element will become an document, so for many unwinds, the result will be enormous db. it will be very hard to handle and they question is what you want to do with them.

corry Over a year ago

I think it couldn't be possible to unwind as many as I want because of $cmp operator. I put a new question regarding this problem stackoverflow.com/questions/39426022/…

Xavier Guihot · Accepted Answer · 2021-12-03 21:24:40Z

5

Starting in Mongo 3.4, you can use $zip to pair your array elements:

// { values: [23, 2, 14], items: ["aa", "bb", "cc"] }
db.collection.aggregate([

  { $project: { x: { $zip: { inputs: ["$values", "$items"] } } } },
  // { x: [[23, "aa"], [2, "bb"], [14, "cc"]] }

  { $unwind: "$x" },
  // { x: [23, "aa"] }
  // { x: [2,  "bb"] }
  // { x: [14, "cc"] }

  { $project: { value: { $first: "$x" }, item: { $last: "$x" } } }
])
// { value: 23, item: "aa" }
// { value: 2,  item: "bb" }
// { value: 14, item: "cc" }

Once your arrays have been $zipped, it's just a matter of $unwinding them before creating documents out of arrays ({ value: { $first: "$x" }, item: { $last: "$x" } }).

answered Dec 3, 2021 at 21:24

Xavier Guihot

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