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Updating documents

$set, $inc, upsert, replaceOne — and the update that silently deletes fields.

By the end: Change existing documents without destroying the parts you did not mean to touch.

1. update() and $set

   MongoDB $set, and the update that silently deletes every other field in your document. The most expensive beginner mistake.

2. $inc and counters

   MongoDB $inc for atomic counters: increment and decrement a numeric field without reading it first.

3. Atomicity: why $inc beats read-modify-write

   Single-document writes in MongoDB are atomic. What that guarantee covers, what it does not, and how to use it correctly.

4. updateOne vs updateMany

   updateOne vs updateMany in MongoDB: which documents each one touches, and why updateOne is the safer default.

5. replaceOne vs $set

   replaceOne vs $set in MongoDB: one swaps the whole document, the other edits fields. Knowing which you called matters.

6. Upsert: insert or update

   MongoDB upsert: update a document if it exists, insert it if it does not, in a single atomic operation.

• $set without clobbering the nested objectwrite
• Atomic guarded decrementwrite
• Upsertwrite