Q26: What is the purpose of $addFields (or $set) in aggregation?
The $addFields stage adds new computed fields or modifies existing fields in passing documents without removing un-mentioned existing fields (unlike $project, which excludes un-specified fields unless explicitly preserved).
Key Takeaways
- Adds or overrides specific document fields.
- Preserves all existing document keys automatically.
Knowledge Check
How does $addFields differ from $project in aggregation?