What problem are you facing?

When .findOne({ field: "value" }) is called repeatedly, there is a constant increase in memory usage.

What driver and relevant dependency versions are you using

Using mongodb from mongoose.

• mongodb: (4.14.0)
• mongoose: (6.7.0)
• bullmq: (3.10.3)

Steps to reproduce?

import { Worker } from 'bullmq'
import mongoose from 'mongoose'

const client = await mongoose.connect(url)
const collection = client.connection.db.collection('collectionName')
new Worker('some-queue', async function() {
  const document = await collection.findOne({ field: "someValue" })
})

My research:

Looking at the source code for .findOne() I see it is calling 

.find().limit(-1).batchSize(1).next()

However, in the mongodb documentation for .batchSize() it mentions to not to specify size of 1:
https://www.mongodb.com/docs/manual/reference/method/cursor.batchSize/

My temporary solution is to .find() without .batchSize(1):

.find().limit(-1).next() // ".findOne()" without memory leak

Memory usage with .findOne()

With .find().limit(-1).next()