Question 115660 in Mongodb Question posted in
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How to get a string in ObjectId in MongoDB v3.6?

I have an aggregation query whose MongoDB repsonse is :

_id: ObjectId('5e822d6c87502b3a9b751786')

I would like to get the string inside the ObjectId which is 5e822d6c87502b3a9b751786.

[ Problem ]

I have searched this question but so far there are only three operators that are capable to do this, namely $toString, $toObjectId, and $convert :

$project: {
      _id: {
        $toString: "$_id"
      }
}

$project: {
      _id: {
        $toObjectId: "$_id"
      }
}

$project: {
      _id: {
        $convert: {
          input: "$_id"
          to: "string"
        }
      }
}

MongoDB v3.6 does not support them if I am not mistaken.
Is there any workaround in MongoDB v3.6 to get a string inside an ObjectId?

Any help is much appreciated 🙂

Tags:

2

Answers


  1. Chosen as BEST ANSWER
    0

    Just want to add ray's answer after some findings.

    Having defined a model in the app, here are some workaround that worked for me:

    1. Promises
    const TestCollection = require('../models/testCollection');

function getStringInObjectId() {
  TestCollection.find().then(t => {
    t.forEach(doc => {
      // Put some logic here...
      console.log('string in ObjectId :', JSON.stringify(doc._id));
    });
  });
}
    1. Async/Await
    const TestCollection = require('../models/testCollection');

async function getStringInObjectId() {
  const t = await TestCollection.find();
  t.forEach(doc => {
    // Put some logic here...
    console.log('string in ObjectId :', JSON.stringify(doc._id));
  });
}

    Same goes to aggregation :

    1. Promises
    const TestCollection = require('../models/testCollection');

function getStringInObjectId() {
  TestCollection.aggregate([
    { $match: { 'name': 'marc' } }, // <- { 'name': 'marc' } is just an example, feel free to change it
    // Put some stages here if required...
  ]).then(t => {
    console.log('string in ObjectId : ', t[0]._id.toString());
  });
}
    1. Async/Await
    const TestCollection = require('../models/testCollection');

async function getStringInObjectId() {
  const t = await TestCollection.aggregate([
    { $match: { 'name': 'marc' } }, // <- { 'name': 'marc' } is just an example, feel free to change it
    // Put some stages here if required...
  ]);
  console.log('string in ObjectId : ', t[0]._id.toString());
}

    Either JSON.stringify() or toString() can be used to convert it to string. Feel free to correct variable names.


  2. 0

    For MongoDB v3.6, as $toString and $convert is not available until v4.0, you may need to resort to JS/application-level access to the _id.

    db.testCollection.insertMany([
{
    "_id": ObjectId("5e822d6c87502b3a9b751786")
}]);

db.testCollection.find().forEach( doc => { 
    // put your logic for process here
    console.log(JSON.stringify(doc._id))
});

    output:

    "5e822d6c87502b3a9b751786"
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