How to use Group By and concat fields as string

Question

I am novice at MongoDB 3.2,

Consider below example,

{ "_id" : "2tLX8ALYfRvbgiurZ", "service" : "GSTR 1" }
{ "_id" : "2tLX8ALYfRvbgiurZ", "service" : "GSTR 2" }
{ "_id" : "2tLX8ALYfRvbgiurZ", "service" : "GSTR 3" }

How can I use Group By _id and create single field with all document fields being concatenated, below is expected output,

{ "_id" : "2tLX8ALYfRvbgiurZ", "service" : "GSTR 1, GSTR 2, GSTR 3" }

I used below but it gives array,

db.getCollection('Clients').aggregate(
      [
        {
            $group : {
             _id : "$_id",
             services :  "$services"
            }
        }
      ]
    ).map( doc =>
      Object.assign(
        doc,
       { "services": doc.services.join(",") }
      )
    );

it gives output as

[{ "_id" : "2tLX8ALYfRvbgiurZ", "service" : "GSTR 1, GSTR 2, GSTR 3" }]

Possible duplicate of [Concat String by Group](https://stackoverflow.com/questions/46841825/concat-string-by-group) — Aakash Verma, Nov 27 '17 at 15:25
Possible duplicate of [Concatenate string values in array in a single field in MongoDB](https://stackoverflow.com/questions/28882552/concatenate-string-values-in-array-in-a-single-field-in-mongodb) — dnickless, Nov 27 '17 at 21:08
@dnickless: The implementation in your suggested link is in mongo db 3.4 — Ankur Soni, Nov 28 '17 at 05:56

score 6 · Answer 1 · edited Apr 02 '20 at 18:31

It's working fine i checked

db.client.insert([
    { "id" : "2tLX8ALYfRvbgiurZ", "service" : "GSTR 1" },
    { "id" : "2tLX8ALYfRvbgiurZ", "service" : "GSTR 2" },
    { "id" : "2tLX8ALYfRvbgiurZ", "service" : "GSTR 3" }
]);

db.getCollection('client').aggregate(
    [
        {
            $group: {
                _id:"$id",
                myfield: {$push: {$concat: ["$service"]}}
            }
        },
        {
            $project:{
                "results": {
                    $reduce: {
                        input: "$myfield",
                        initialValue: '',
                        in: {$concat: ["$$value", ", ", "$$this"]}
                    }
                }
            }
        }
    ]
);

You can also add ` '$cond': [ { '$eq': [ '$$value', '' ] }, '', ', ' ]` in the `$concat` to omit any leading ", " — user2402616, May 19 '21 at 16:23

Israel Zinc · Accepted Answer · 2017-11-28T06:16:02.370

0

docIwant = db.getCollection('Clients').aggregate(
      [
        {
            $group : {
             _id : "$_id",
             services :  "$services"
            }
        }
      ]
    ).map( doc =>
      Object.assign(
        doc,
       { "services": doc.services.join(",") }
      )
    );
return docIwant[0].

edited Nov 28 '17 at 06:16

answered Nov 28 '17 at 06:02

Israel Zinc

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`$push` creates array, I need concatenated String. – Ankur Soni Nov 28 '17 at 06:06
I would risk to say that that's not possible using mongodb solo can't be used for this task. I did a fast research in concatenations using projections but I could not find an answer for that. Maybe you could use your favorite programming language to concatenate after the query. If it's in python I can give you a hand. – Israel Zinc Nov 28 '17 at 06:12
@isael.zinc : thanks a lot. I am using javascript :) – Ankur Soni Nov 28 '17 at 06:22
If my answer answered your question, please accept it. – Israel Zinc Nov 28 '17 at 06:33

How to use Group By and concat fields as string

2 Answers2