MongoDB aggregation: Aggregate array into keyed objects

Question

I'd like to flatten document array into a keyed objects using the aggregation functionality. Here’s an example of my documents:

[ 
    {
        "_id": 1,
        "texts": [
            { "language": "english", "text": "hello" },
            { "language": "german", "text": "hallo" },
            { "language": "french", "text": "bonjour" }
        ]
    }, …
]

Expected result:

[
     {
        "_id": 1,
        "texts": {
            "english": "hello",
            "german": "hallo",
            "french": "bonjour"
        }
    }, …
]

I’ve looked at different operators, e.g. $map, but this seems to be focued on transforming array to array. I’d probably need a JS equivalent of $reduce, but couldn’t find a way of accumulating my values into an object.

Any ideas?

mickl · Accepted Answer · 2018-08-23 17:28:33Z

4

You need $map and $arrayToObject which converts an array of k-v pairs into single object:

db.col.aggregate([
    {
        $addFields: {
            texts: {
                $arrayToObject: {
                    $map: {
                        input: "$texts",
                        as: "text",
                        in: {
                            k: "$$text.language",
                            v: "$$text.text"
                        }
                    }
                }
            }
        }
    }
])

answered Aug 23, 2018 at 17:28

mickl

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Comments

s7vr · Accepted Answer · 2018-08-23 18:07:08Z

3

You can also use $zip with $arrayToObject.

Something like

db.col.aggregate([{
  "$project": {
    "texts":{
     "$arrayToObject":{
       "$zip": {
        "inputs": [
          "$texts.language",
          "$texts.text"
        ]
      }
    }
  }}
}])

answered Aug 23, 2018 at 18:07

s7vr

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1 Comment

qqilihq Over a year ago

Cool, even more concise. Will give it a try tomorrow and give feedback. Thx!

dnickless · Accepted Answer · 2018-08-23 19:57:03Z

2

You can try this using $map and $arrayToObject aggregation in mongodb 3.4.4 and above

Just convert language field with key(k) and text with value(v) using $map and transform the array to object using $arrayToObject

db.collection.aggregate([
  { "$addFields": {
    "texts": {
      "$map": {
        "input": "$texts",
        "in": {
          "k": "$$this.language",
          "v": "$$this.text"
        }
      }
    }
  }},
  { "$addFields": {
    "texts": { "$arrayToObject": "$texts" }
  }}
])

edited Aug 23, 2018 at 19:57

dnickless

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answered Aug 23, 2018 at 17:28

Ashh

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7 Comments

qqilihq Over a year ago

Many thanks for the very rapid answer! +1 Both were exactly simultaneous and work equally well. The reason I'm accepting mickl’s answer instead of yours is, that I prefer having everything in a single pipeline stage. Cheers!

qqilihq Over a year ago

Interesting update: I'm actually still running MongoDB 3.4.6 -- still the aggregation works just fine.

Ashh Over a year ago

not sure why so... but $arrayToObject has been introduced in mongodb 3.6

dnickless Over a year ago

That's how they announced it. Mind you, that's not true. ;) It has been around since v3.4.4: docs.mongodb.com/v3.4/reference/operator/aggregation/…

Ashh Over a year ago

@dnickless oops!!! I had mistaken... thanks for comeuppance me... updated the answer

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