How to get a string in ObjectId in MongoDB v3.6? - mongodb

Question

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 :)

Answer 1

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"

Answer 2

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:
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));
});
});
}
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 :
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());
});
}
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.