Which operator can be utilized to rename fields in a document?

The operator that is utilized to rename fields in a document is $rename. This operator is specifically designed for this purpose within MongoDB's update operations. When you use the $rename operator, you can change the name of an existing field to a new one in documents that match a specified query. For example, if you need to change the field name "oldName" to "newName" in documents, the syntax would be:

db.collection.updateMany(

{ /* criteria to select documents */ },

{ $rename: { "oldName": "newName" } }

);

By employing this operator, you maintain the existing data under the new field name while removing the old field.

The other operators listed serve different purposes. The $set operator is used for updating the value of a field but does not change the field name itself. The $project operator is used in aggregation frameworks to reshape documents and is not applicable for renaming fields directly. Lastly, the $out operator is used to write the results of a pipeline to a specified collection and does not involve renaming fields. Thus, $rename is the only operator that directly addresses the requirement to rename fields in a document.