Mongodb - mongo query : How can a direct key-value pair be placed from a nested array in a single stage pipeline and without the use of $unwind?

SonuSejode
July 13, 2023
162 views
1 vote
2 Answers

I have something like this.

{
  "priceDetails": [
    {
      "cities": [
        {
          "cityId": "64a297a8bb1dc440ccef6c3c",
          "name": "Bhopal",
          "prices": "1200",
          "commissionType": "PERCENTAGE",
          "commissionValue": "20",
          "_id": "64a2990b344e735476fb54ca"
        }
      ]
    }
  ]
}

Need something like this.

{
  "priceDetails": {
    "cityId": "64a297a8bb1dc440ccef6c3c",
    "name": "Bhopal",
    "prices": "1200",
    "commissionType": "PERCENTAGE",
    "commissionValue": "20",
    "_id": "64a2990b344e735476fb54ca",
  }
}

I tried $unwind, but it uses a lot of database processing power to filter and retrieve the data.

I’m using node v18.7.0 & mongo version v4.4

I’m baffled at this point, any help would be greatly appreciated. Cheers!

Answers

- nimrodserok
- July 12, 2023 at 3:00 pm
- 0 votes
0
One option is to use a simple projection:
```
db.collection.find(
  {},
  {
    _id: 0,
    priceDetails: {$first: "$priceDetails.cities"}
  }
)
```
See how it works on the playground example
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- siddharthverma
- July 13, 2023 at 11:57 am
- 0 votes
0
In your present pipeline for aggregating data, you can utilise $set and $arrayElemAt.

To modify the response, use $set, and to retrieve an element from an array, use $arrayElemAt.
```
{
  $set: {
    priceDetails: {
      $arrayElemAt: [
        {
          $arrayElemAt: ["$priceDetails.cities", 0]
        },
        0
      ]
    }
  }
}
```
The first index of the array will be set by inner $arrayElemAt. in the outer array’s 0 index. The outer $arrayElemAt will then act in a manner similar to that of the inner $arrayElemAt.

Consult the document for more details.

$set : https://www.mongodb.com/docs/manual/reference/operator/update/set/#definition

$arrayElemAt : https://www.mongodb.com/docs/manual/reference/operator/aggregation/arrayElemAt/

I hope that this may be useful to you or someone else.
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