Return keys of an array field from MongoDB

Question

Below is the mongodb collection sample data. I'm trying to fire a query which will return and array of a property in my collection.

Sample Docs :

{
    "_id" : ObjectId("5e940d6c2f804ab99b24a633"),
    "accountId" : ObjectId("7e1c1180d59de1704ce43557"),
    "description":"some desc",
    "configs": {},
    "dependencies" : [ 
        {
            "commute" : {},
            "distance" : {},
            "support":{}
        }
    ]
}

Expected output :

{
    [0]:commute
    [1]:distance
    [2]:support
}

I've tried to get the response and iterate the dependencies object also I tried using es6 to convert it to an array object. But it is an expensive operation as this list would be really large. If at all there exists and such approach in Mongo to convert the response into array

So you just want the names of keys in dependencies field into an array ? — whoami - fakeFaceTrueSoul
– whoami - fakeFaceTrueSoul, Commented Apr 13, 2020 at 15:07
@whoami - yes, whatever keys are there Im trying get it as an array — The JOKER
– The JOKER, Commented Apr 13, 2020 at 15:19

whoami - fakeFaceTrueSoul · Accepted Answer · 2020-04-13 16:06:28Z

You can try below aggregation query :

db.collection.aggregate([
  {
    $project: {
      _id: 0,
      dependenciesKeys: {
        $reduce: {
          input: "$dependencies", // Iterate over 'dependencies' array
          initialValue: [],
          in: {
            $concatArrays: [ /** concat each array returned by map with holding 'value */'
              "$$value",
              {
                $map: {
                  input: {
                    $objectToArray: "$$this" /** Convert each object in 'dependencies' to array [{k:...,v:...},{k:...,v:...}] & iterate on each object */
                  },
                  in: "$$this.k" // Just return keys from each object
                }
              }
            ]
          }
        }
      }
    }
  }
])

Test : MongoDB-Playground

Ref : $reduce , $map , $concatArrays , $objectToArray & $project

Ahmad Shaarawi · Accepted Answer · 2020-04-13 12:19:43Z

0

I think you should try Object to array, which is an aggregation pipeline operator.

Also the Aggregation Pipeline is pretty useful concept, it makes you create a pipeline of operations that process your data sequentially. You can imagine it as following; you create a steps 'Pipeline' every step make a certain operation 'Aggregate operation' in your data, which results to some certain data shape,like an array as you want.

answered Apr 13, 2020 at 12:19

Ahmad Shaarawi

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