# ObjectId generation inside aggregation and update expressions > Source-anchored research for ObjectId generation inside aggregation and update expressions. Run status: COMPLETED. Parent: [MongoDB Aggregation Pipeline](https://llms-explorer.com/tree/mongodb-aggregation-pipeline/) · 3 facets · 10 facts · page: https://llms-explorer.com/tree/objectid-generation-inside-aggregation-and-update-expressions/ ## How it works - Using $createObjectId in an update does not remove the immutability of an existing stored _id. Generate an ordinary identifier field when updating stored documents; the official example that supplies a new _id operates on a view projection. (medium confidence) — [source](https://www.mongodb.com/docs/manual/reference/operator/aggregation/createObjectId/) - A first-hand experiment using MongoDB 8.3.11 reports that $createObjectId fails when FCV is 8.2 and succeeds after FCV is set to 8.3. Treat this as a reported compatibility observation, pending independent runtime reproduction. (low confidence) (verified as of 2026-10-11) — [source](https://dev.to/alexgeorgiev17/mongodb-83s-arrayindexas-cuts-a-map-with-index-workarounds-overhead-by-up-to-31-409p) ## Problems, failure modes and limitations - The wire expression is {$createObjectId: {}}. Only an empty object is accepted; other arguments fail with FailedToParse. A published first-hand experiment reproduces rejection of {x: 1}. (medium confidence) — [source](https://www.mongodb.com/docs/manual/reference/operator/aggregation/createObjectId/) ## Facts and statements - MongoDB introduced $createObjectId in 8.3 to generate new random ObjectId values within aggregation pipelines and expression-based updates. (medium confidence) — [source](https://www.mongodb.com/docs/manual/reference/operator/aggregation/createObjectId/) - $createObjectId generates a value without a source-value input; $toObjectId converts an existing value. NoSQLBooster documents this distinction alongside its $.createObjectId() helper. (medium confidence) — [source](https://www.mongodb.com/docs/manual/reference/operator/aggregation/createObjectId/) - Generation occurs for output documents: the official view example assigns _id: {$createObjectId: {}}, and the C# LINQ example assigns a new NewId for each output document using Mql.CreateObjectId(). (medium confidence) — [source](https://www.mongodb.com/docs/manual/reference/operator/aggregation/createObjectId/) - A supported update pattern is db.items.updateMany({}, [{$set: {generatedId: {$createObjectId: {}}}}]); this combines the documented expression-based-update support with the documented pipeline $set stage. This is a documentation-derived example, not a locally executed test. (medium confidence) — [source](https://www.mongodb.com/docs/manual/reference/operator/aggregation/createObjectId/) - In a Stack Overflow exchange, a commenter reported the same ObjectId in all array objects. The answer author acknowledged the issue and updated the answer, whose code uses $function with a JavaScript body returning an ObjectId for each mapped element. (low confidence) — [source](https://api.stackexchange.com/2.3/questions/68119736/answers?site=stackoverflow&filter=withbody) - NoSQLBooster 11.1 exposes $.createObjectId() as an aggregation expression helper; its release announcement dates this support to September 30, 2026. (low confidence) (verified as of 2026-09-30) — [source](https://www.nosqlbooster.com/blog/announcing-nosqlbooster-11-1/) - The operator’s implementation ticket lists fix version 8.3.0-rc0 and separately links an SBE-support task. The separate task establishes an implementation question; it does not by itself establish current performance or execution-engine behavior. (medium confidence) — [source](https://jira.mongodb.org/browse/SERVER-106452)