You use the bucket pattern and append readings with `$push` until a bucket holds 200 items, then start a new bucket. Which single update operation is the idiomatic way to atomically append AND roll over to a new bucket only when there is room?
db.buckets.updateOne(
{ sensorId: 7, count: { $lt: 200 } },
{ $push: { readings: r }, $inc: { count: 1 } },
{ upsert: true }
)