Concat strings in nested json with other string in MongoDB

Question

I am trying an aggregation in MongoDB in order to convert:

[
  {
    "_id": ObjectId("5a934e000102030405000000"),
    "acronym": "MC",
    "tickers": [
      {
        "name": "Sensor1-front",
        "symbol": "s1f"
      },
      {
        "name": "Sensor1-back",
        "symbol": "s1b"
      }
    ]
  }
]

into something like this:

['s1f.MC','s1b.MC']

Where each symbol in tickers is concat to acronym.

I am trying it with $concat, $group, etc but I am not going anywhere.

Thanks!

I'm not sure but may be you could try $set, $concat and $merge. — Medo Paw
– Medo Paw, Commented Oct 12, 2021 at 1:13

varman · Accepted Answer · 2021-10-11 09:26:46Z

3

You can use reduce

db.collection.aggregate([
  {
    $addFields: {
      newField: {
        "$reduce": {
          "input": "$tickers",
          "initialValue": [],
          "in": {
            "$concatArrays": [
              [
                {
                  "$concat": [
                    "$$this.symbol",
                    ".",
                    "$acronym"
                  ]
                }
              ],
              "$$value"
            ]
          }
        }
      }
    }
  }
])

Working Mongo playground

answered Oct 11, 2021 at 9:26

varman

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2 Comments

chridam Over a year ago

You can also use $map as follows: mongoplayground.net/p/l5J7U5ddBS8

ignareyesa Over a year ago

Yeah, using $map is also a good solution!

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