What is the purpose of the $set stage in an aggregation pipeline?

The $set stage in an aggregation pipeline is specifically designed for adding new fields or modifying existing fields within documents as they pass through the pipeline. When you utilize $set, you can introduce new data or change the values of current fields based on computations, existing field values, or even static values. This stage is flexible and robust, allowing for the transformation of documents to meet the needs of your data processing requirements.

For instance, you might want to calculate a new field that represents a combination of two existing fields or update a field based on certain conditions. The primary purpose of $set is to ensure that the resulting documents contain the structure and data required for subsequent stages in the pipeline.

In contrast, combining documents from multiple collections is typically handled by stages such as $lookup, counting documents would involve stages like $count, and filtering is done using the $match stage. Each of these serves different roles within the aggregation framework, reinforcing the specific utility of the $set stage in modifying and enhancing document contents.