A little context: I have a document for each user that contains an array

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Asked: May 18, 20262026-05-18T22:28:35+00:00 2026-05-18T22:28:35+00:00

A little context: I have a document for each user that contains an array

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A little context: I have a document for each user that contains an array with latest 20 events related to a user. As MongoDB does not have this feature(to cap arrays inside a document), I will push my event and pop the latest one.

My problem: initializing the document(aka filling array with nulls). I want to atomically:

create document containing an array with 20 null values and push one value, if document does not exist

or

update document (push one value in array), if document exists

Do you have any other suggestions? A hack I thought about would be to declare a index with :unique and :dropDups, and to always make an initialization insert.

Related to: MongoDB fixed size array implementation