How to generate a MongoDB ObjectId from a date in Bash

A little known fact about MongoDB object IDs is that they encode the documents creation date. This means that you don't have to create an index on a dedicated date field, because the _id field is automatically indexed. Let's generate an ObjectId with nothing else than a command line, sprinkled with some black magic:

# 1. get the date 3 months ago
$ date=$(date --date="3 months ago" +%s)

# 2. generate the ObjectId by converting the date to hex and fill the rest with 0s
$ echo "$(printf "%x" ${date})0000000000000000"
5ca378720000000000000000

# 3. query the DB for documents according to creation date
$ mongo
>> use mydb
>> db.collection.find({_id:{$gt: ObjectId("5cab57740000000000000000")}})

1. date has a nifty feature, where you can provide a human readable, relative string called a date string, and it will get back to you with the date at that time.

2. Mongo object IDs consist of 12 byte hexadecimal strings. The first 4 describe the number of seconds since the epoch - the star of our show, the next 5 is a random value, and the last 3 bytes describe a counter. Since only the first 4 bytes are storing the date, we can fill the rest with zeros.

   We convert our date (expressed in seconds since the epoch) to Hex, by using the built-in printf function and the %x (unsigned hexadecimal) format.

3. We use the generated object ID to execute the gt (greater than) query, and as a result, we will get all the documents which were created after the given date.

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Now, why would you need this?

If you don't have an index on a date field, but you need to filter out documents with a date boundary. For example:

• deleting all documents older than 30 days
• archiving all documents older than 6 months and then removing the data to save MongoDB disk space [^1]

[^1]: deleting documents will not free up the disk on the host - Mongo reserves the newly available free space for new documents. Starting from Mongo 3.2, when you compact a collection, then the OS will regain the free space.