how to $project ObjectId to string value in mongodb aggregate?

Question

Is there an operator I could use in aggregate function to get a string instead of ObjectId in response?

db.something.aggregate([
  { "$match": { "property": { "$exists": true } } },
  { "$project": { "stringId": "$_id.???" } }
])

No there isn't. The aggregation framework does not re-cast any data types ( except to string from numeric via `$substr` and numeric to Date or Date to numeric is basically possible with trickery ). Why would you think this is needed anyway? It's fairly simple in most languages to write the `ObjectId` value as a string anyway. — Blakes Seven, Mar 17 '16 at 12:04
@BlakesSeven There is always why :) Because of simplicity. Of course you can post process it. But it would be nice if I could just pass the result to other service/party who requires plain string. ... — matus, Mar 17 '16 at 12:16
Well like I said, with most languages the BSON is just cast to local types. And for "most" languages, there is no work at all required to simply output as a string. Personally I prefer the extended JSON output form i.e `{ "$oid": "56ea9e8bb1e015d13b376db5" }` since at least that lets a remote client know that the data is indeed an `ObjectId` so it can parse and cast correctly itself. That's a good thing, especially considering that storage is **half** the bytes of the string length. — Blakes Seven, Mar 17 '16 at 12:32

Ashh · Accepted Answer · 2018-11-05T06:59:42.270

91

Mongodb 4.0 has introduced $toString aggregation operator. So, Now you can easily convert ObjectId to string

db.collection.aggregate([
  {
    $project: {
      _id: {
        $toString: "$_id"
      }
    }
  }
])

OR vice versa using $toObjectId aggregation

db.collection.aggregate([
  {
    $project: {
      _id: {
        $toObjectId: "$_id"
      }
    }
  }
])

edited Nov 05 '18 at 06:59

answered Jul 08 '18 at 10:21

Ashh

44,693
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16

How about in Mongodb version 3.6? – Charlie Oct 25 '18 at 14:38
4

@Charlie No you can't. You have to upgrade to 4.0 and that's what always been intresting about the new versions. – Ashh Oct 25 '18 at 14:43
But how to do this in version 3.9 ? – Boat Jan 21 '20 at 13:14
@Boat Mongodb has not introduced any 3.9 version. Are you talking about any driver version? or mongodb 3.6.9? – Ashh Jan 21 '20 at 13:16
@Ashh I was referring to mongodb 3.6.9 – Boat Jan 22 '20 at 09:36
1

@Boat Then have a look at second comment. – Ashh Jan 22 '20 at 10:07

score 11 · Answer 2 · edited Feb 01 '18 at 07:29

There is no Direct Operator in aggregate function to get String from ObjectId.

After version 2.6 You can use ObjectId.toString() method to convert your ObjectId to string. First you match and project your ObjectID. Then you can convert this object ID to string by using ObjectID.toString().

db.something.aggregate([{"$match":{'property': {$exists:true}}},{"$project":{"_id":1}}])

And then use resulting Object and get the string as response using ObjectID.tostring()

Edit: You can access the str attribute of the object id using

ObjectId("507f191e810c19729de860ea").str

source: mongodb docs

score -17 · Answer 3 · edited Aug 02 '16 at 22:00

-17

You can do it inline using the $concat operator:

db.something.aggregate(
    [
        { $match :
            { 'property' :
                { $exists:true }
            }
        },
        { $project:
            { stringId:
                { $concat: [ ObjectId().str ] }
            }
        }
    ]
)

edited Aug 02 '16 at 22:00

Matt Clark

27,671
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answered Aug 02 '16 at 20:47

user1260158

1
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For me, this doesn't make "stringId" the ObjectId of the record. It makes it a stringified version of a brand new ObjectId. In particular, it's the same value for all the documents returned. So I don't think this is right. – mcdave Oct 26 '16 at 17:29
lol fail ...... – Nicholas Hamilton Aug 20 '22 at 22:01

how to $project ObjectId to string value in mongodb aggregate?

3 Answers3

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