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

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 two operators which are $toString and $toObjectId :

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

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

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 :)

CodePudding user response：

Use $convert

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

Playground

CodePudding user response：

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"