midMongoDB
What is an 'Upsert' operation?
Updated May 1, 2026
Short answer
An upsert is a hybrid operation that updates a document if it exists, or inserts it if it doesn't.
Deep explanation
It is enabled by setting the 'upsert' option to true in update methods. It prevents 'document not found' errors in many-to-one sync patterns.
Real-world example
Recording the 'last login' time for a user regardless of whether it's their first time.
Common mistakes
- Assuming the query filter will be ignored during insertion
- the filter fields are used to build the new doc.
Follow-up questions
- What happens if multiple docs match an upsert?