filter Items of Array by Index in Aggregation Pipeline

Question

I have mongodb Database and I have this document structure :

   {
     "_id": ObjectId("2145214545ffff"),
     "arr":[a , b , c , d , e , f , g , h , i, j]
   }

I want to execute aggregation pipeline , which gives me this result:

 {
         "_id": ObjectId("2145214545ffff"),
         "arr":[a , d , g , j ]
       }

So what i need is kind of filtering on the array's items , which give me the 1st , 4th , 7th and so on items. Thx in advanced

What have you tried so far?

Hoppo
– Hoppo

2020-06-04 18:58:37 +00:00
Commented Jun 4, 2020 at 18:58 — Hoppo
– Hoppo, Commented Jun 4, 2020 at 18:58

ngShravil.py · Accepted Answer · 2020-06-05 10:52:38Z

1

You need to use $map and $range operators.

The following query will be helpful:

db.collection.aggregate([
  {
    $project: {
      arr: {
        $map: {
          input: {
            $range: [
              0,
              {
                $size: "$arr"
              },
              3
            ]
          },
          as: "a",
          in: {
            $arrayElemAt: [
              "$arr",
              "$$a"
            ]
          }
        }
      }
    }
  }
])

MongoPlayGroundLink

edited Jun 5, 2020 at 10:52

answered Jun 4, 2020 at 18:38

ngShravil.py

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2 Comments

ngShravil.py Over a year ago

Hey @MoOmar, was this helpful?

Mo Omar Over a year ago

Hey @ngShravil.py , this was nice,Thank you very much. Also The performance is great. I used reduce operator , it was working but slower. So map operator is nice for this use case

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