Mongodb : How to convert a field inside a nested array of object

Question

Is there any way to convert a field inside a nested array of object in a query ?

Here is a simple exemple :

My collection :

{ _id: 1, quizzes: [ { _id: 1, question: "bla bla 1"}, { _id: 2, question: "bla bla 2"}, ... ] },
{ _id: 2, quizzes: [ { _id: 1, question: "bla bla 1"}, ... ] }

Currently, my _id are ObjectId, I want to convert all of them including quizzes._id to string.

So here is the expected final result

{ _id: "1", quizzes: [ { _id: "1", question: "bla bla 1"}, { _id: "2", question: "bla bla 2"}, ... ] },
{ _id: "2", quizzes: [ { _id: "1", question: "bla bla 1"}, ... ] }

Here is what I got so far :

db.collection.aggregate([
{
        $addFields: {
            _id: { $toString: "$_id" }, // OK
            quizzes: { $map: { input: "$quizzes", in: { $toString: '$$this._id' }}}
])

But this is wrong, each quizze Object is fully transformed to a string, not just the id.

I am using Mongo 3.4.

mickl · Accepted Answer · 2020-07-01 10:32:27Z

1

You need to return an object so instead of in: { $toString: '$$this._id' } it should be:

{ _id: { $toString: '$$this._id' }, question: "$$this.question" }

try:

db.collection.aggregate([
    {
        $addFields: {
            _id: { $toString: "$_id" },
            quizzes: { $map: { input: "$quizzes", in: { _id: { $toString: '$$this._id' }, question: "$$this.question" } }}
        }
    }
])

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answered Jul 1, 2020 at 10:32

mickl

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2 Comments

loonis Over a year ago

hello mickl, thank you for your quick answer. Your solution works. But my real object "quizze" has a lot of fields. Moreover, more can be added in the futur. Is there a way to avoid listing all of them in my in: expression ?

mickl Over a year ago

@loonis you can use $mergeObjects like here: stackoverflow.com/questions/53934300/…

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