update aggregate doesn't modify all the documents

Question

Sample data:

{
  _id: ,
  instances: [
    {
     instance_id: ObjectId()
    }
  ]
}

I need to convert to ObjectId to String format in the instances array.

I came up with the below query

db.test.update(  
  {'_id': {'$in': ['1054605|2347', '1053095|2404']}},
  [ 
    {
    $set: {
      i: {
        $map: {
          input: "$instances",
          as: "instance",
          in: {
            $mergeObjects: [
              "$$instance",
              {
                instance_id: {
                  $cond: {
                    if: {
                     $eq:[ { $type: "$$instance.instance_id"} , "objectId"]
                  },
                  
                  then: {
                      $convert:{ 
                          input: "$$instance.instance_id", 
                          to: "string"
                     }
                 },
                    else: "$$instance.instance_id"
                  }
                }
              }
            ]
          }
        }
      }
    }
  }
  ])

But the above query works for only one document. But it doesn't work on multiple documents, it simply says updated 0 documents.

Any suggestions please?

Answer 1

You are using the update method:

By default, the db.collection.update() method updates a single document. Include the option multi: true to update all documents that match the query criteria.

All you have to do is add the multi: true flag to the update options, like so:

db.collection.update(
   <query>,
   <update>,
   {
     multi: true,
   }
)