How can I concat an array of integer in MongoDB aggregation method?

Question

I have a field in mongodb document which is an array of integer as like:

import_ids=[5200, 4710, 100]

I want this array as double ## separated string, So that expected result would be

import_hashed="5200##4710##100"

I have tried with following code in $project pipeline of aggregation method.

 {
     $projct:{
      import_hashed:{
           $reduce:{
              input:"$import_ids",
              initialValue:"",
              in:{$concat:["$$value", "##", "$$this"]}
             }
         }
      } 
 }

But no result found and no erros too!

Ashh · Accepted Answer · 2018-10-17 17:23:50Z

2

You can try below aggregation

You can use $toLower aggregation to convert integer to string or $toString if you are using mongodb 4.0

db.collection.aggregate([
  { "$project": {
    "import_hashed": {
      "$let": {
        "vars": {
          "key": {
            "$reduce": {
              "input": "$import_ids",
              "initialValue": "",
              "in": { "$concat": ["$$value", "##", { "$toLower": "$$this" }] }
            }
          }
        },
        "in": { "$substrCP": ["$$key", 2, { "$strLenCP": "$$key" }] }
      }
    }
  }}
])

Output

{ "import_hashed": "5200##4710##100 }

edited Oct 17, 2018 at 17:23

answered Oct 17, 2018 at 7:46

Ashh

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1 Comment

Shamim Hossain Over a year ago

Thanks for quick response its working, But result is not perfect. I am getting following result which are single array integer import_hashed: "##287" and from multiple array element import_hashed: "##5200##4710##100" I don't expect first hash in the resulted string.

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