MongoDB Native Driver Patterns

Quick Guide: Talk to MongoDB through the official mongodb driver with no schema layer in between. Create ONE MongoClient per process and reuse it -- it owns the connection pool. Type collections with a generic: db.collection<UserDoc>("users"). Write operations return acknowledgements, never documents. find() returns a lazy cursor; stream it with for await instead of toArray() for anything unbounded. Put $match first in every pipeline so it can use an index. Verify indexes with explain("executionStats") rather than assuming. Transactions need a replica set, a session on every operation, and a callback that can safely run twice.

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CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST create exactly ONE MongoClient per process and reuse it -- the client owns a connection pool, so constructing one per request opens a new pool per request and exhausts the server's connection limit)

(You MUST pass { session } to EVERY operation inside a transaction -- an operation without it silently runs outside the transaction and is not rolled back)

(You MUST write withTransaction callbacks to be safely re-runnable -- the driver retries them on transient errors, so any side effect outside the transaction happens more than once)

(You MUST iterate or close every cursor you open -- an abandoned cursor holds server-side resources until it times out)