MongoDB Query: How can I aggregate an array of objects as a string

Question

I have an array of objects where I want to make a string concatenating all of the same attributes from the array. Example:

{
    _id: 123,
    example_document: true,
    people: [
        {
            name: "John",
            age: 18
        }, {
            name: "Clint",
            age: 20
        }
    ]
}

And I wanna make a query where my result would be:

{
    _id: 123,
    example_document: true,
    people: [
        {
            name: "John",
            age: 18
        }, {
            name: "Clint",
            age: 20
        }
    ],
    concat_names: "John, Clint"
}

I think aggregate is the path I should take, but I'm not being able to find a way of getting a string out of this, only concat array elements into another array. Anyone could help?

Tom Slabbaert · Accepted Answer · 2022-01-06 14:44:04Z

1

You can use $concat combined with $reduce to achieve this, like so:

db.collection.aggregate([
  {
    $addFields: {
      concat_names: {
        $reduce: {
          input: "$people",
          initialValue: "",
          in: {
            $concat: [
              "$$value",
              {
                $cond: [
                  {
                    $eq: [
                      {
                        "$strLenCP": "$$value"
                      },
                      0
                    ]
                  },
                  "",
                  ", "
                ]
              },
              "$$this.name"
            ]
          }
        }
      }
    }
  }
])

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answered Jan 6, 2022 at 14:44

Tom Slabbaert

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1 Comment

pliniocf Over a year ago

Thx a lot, it solved it

Liberateur · Accepted Answer · 2022-01-06 14:49:42Z

0

You can use aggregation operators $project, and $map:

db.collection.aggregate([
{
    $project:
    {
        concat_names:
        {
            $map:
            {
                input: "$people",
                as: "i",
                in: "$$i.name"
            }
        }
    }
}])

edited Jan 6, 2022 at 14:49

answered Jan 6, 2022 at 14:24

Liberateur

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2 Comments

pliniocf Over a year ago

Can you actually use $concat with a map? I'm getting an error telling me that concat can only be used with strings, and map returns an array

Liberateur Over a year ago

Oh, concat or maybe concatArrays

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