mongodb remove documents older than period of time but with no date attribute - mongodb

Question

we're trying to remove documents older than 3 months in a specific collection.
There's no TTL configured on this collection and no single date/time attribute on those documents.
How can I remove those old documents anyway? is there any script I could run to make it automatically?
thanks

Answer 1

Assuming you did not generate your own _id field the ObjectId contains a timestamp, from the docs:
The 12-byte ObjectId value consists of: ...
a 4-byte timestamp value, representing the ObjectId's creation, measured in seconds since the Unix epoch
So if you're using Mongo version 4.0+ you could use $toDate, match the documents and overwrite the current collection using $out
db.collection.aggregate([
{
$addFields: {
shouldKeep: {
$lt: [
{
$subtract: [
"$$NOW",
{
$toDate: "$_id"
}
]
},
7776000// 90 days in seconds
]
}
}
},
{
$match: {
shouldKeep: true
}
},
{
$project: {
shouldKeep: 0
}
},
{
out: "curr_collection"
}
])
Mongo Playground
Mind you this is a POC example however this does not deal with many issues, such as timezones. exact month start and ending ( currently it calculates 90 days back ) and more.
Not mentioning that using $out on a large collection contains a lot of overhead.
My recommendation is to paginate the results and do this in code.
For nodejs for example you can use ObjectId's getTimestamp method, like so: (pseudo code)
const someDocuments = [...];
for each document:
const timestamp = document._id.getTimestamp();
if (timestamp < 3 months ago) delete document.
Now in code you can handle timezones, month start dates and scale issues relatively easily.