Reference Existing fields in Update Query MongoDB

February 27, 2024

I am trying to add a new field to my collection in MongoDB (This collection has around 40k documents) using existing fields in the document, I’m running the below update query

db.myCollection.updateMany( 
{ },
[
    { $set : { newField : "some_extra_string" + "$existingField1" + "other_extra_string" + "$existingField2" } }
])

However the newField is getting updated as

"newField" : "some_extra_string$existingField1other_extra_string$existingField2"

It’s not referencing the existing field, rather appending the string literals "$existingField1"/"$existingField2".

I’m using the MongoDB Documentation as my source : https://www.mongodb.com/docs/manual/reference/method/db.collection.updateMany/#example-1–update-with-aggregation-pipeline-using-existing-fields

Here in the example, they’re referencing existing fields using the dollar sign, what am I doing wrong here?

>Solution :

Currently, you are doing the string concatenation with plain text.

Instead, you need the $concat operator to concat strings with the reference field(s).

db.myCollection.updateMany({},
[
  {
    $set: {
      newField: {
        $concat: [
          "some_extra_string",
          "$existingField1",
          "other_extra_string",
          "$existingField2"
        ]
      }
    }
  }
])

Demo @ Mongo Playground