Size of nested array in aggregation projection

Question

I have a "Quiz" model, where each quiz document has some questions and an array of answers in them.

Example document structure (Quiz.findOne()):

_id: ObjectId("611478ac34dde61f28dbe4db"),
name: "Quiz 1",
questions: [
    {
       text: "Question 1",
       answers: ["a", "b", "c"],
    },
    {
       text: "Question 2",
       answers: ["m", "n", "o", "p"],
    },
    ...
    ...
    {
       text: "Question 1000",
       answer: ["a", "c", "e", "f"],
    },
 ]

I am selecting some particular questions by index, using MongoDB aggregation.

Aggregation code:

Quiz.aggregate([
                {
                    $match: { _id: "611478ac34dde61f28dbe4db" },
                },
                {
                    $addFields: {
                        questions: {
                            $map: {
                                input: [0, 1], //Choosing the questions at index 0 and 1 (can be any other index)
                                as: "i",
                                in: {
                                    $arrayElemAt: ["$questions", "$$i"],
                                },
                            },
                        },
                    },
                },
                {
                    $project: {
                        "questions.answers": 1,
                    },
                },
            ])

Output comes as:

[{
_id: "611478ac34dde61f28dbe4db",
questions: [
    {
       answers: ["a", "b", "c"],
    },
    {
       answers: ["m", "n", "o", "p"],
    },
 ]
}]

How can I convert this output to show only the "$size" of the array?

Expected output:

[{
_id: "611478ac34dde61f28dbe4db",
questions: [
    {
       answers: 3, //Had 3 answers: a, b, c
    },
    {
       answers: 4, //Had 4 answers: m, n, o, p
    },
 ]
}]

NeNaD · Accepted Answer · 2021-08-22 23:52:29Z

1

You can add new aggregation stage where you can use $map and $size pipeline operators, like this:

db.collection.aggregate([
  {
    $project: {
      "questions": {
        $map: {
          input: "$questions",
          as: "question",
          in: {
            answers: {
              $size: "$$question.answers"
            }
          }
        }
      }
    }
  }
])

Here is the working example: https://mongoplayground.net/p/0eyAlMWai0A

edited Aug 22, 2021 at 23:52

answered Aug 22, 2021 at 23:47

NeNaD

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1 Comment

Chique Over a year ago

Works! Appreciate your help.

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