ObjectId

BSON Types — MongoDB Manual https://docs.mongodb.com/manual/reference/bson-types/#objectid

ObjectId

ObjectIds are small, likely unique, fast to generate, and ordered. ObjectId values consist of 12 bytes, where the first four bytes are a timestamp that reflect the ObjectId’s creation. Specifically:

a 4-byte value representing the seconds since the Unix epoch,
a 5-byte random value, and
a 3-byte counter, starting with a random value.

In MongoDB, each document stored in a collection requires a unique _id field that acts as a primary key. If an inserted document omits the _id field, the MongoDB driver automatically generates an ObjectId for the _id field.

This also applies to documents inserted through update operations with upsert: true.

MongoDB clients should add an _id field with a unique ObjectId. Using ObjectIds for the _id field provides the following additional benefits:

in the mongo shell, you can access the creation time of the ObjectId, using the ObjectId.getTimestamp() method.
sorting on an _id field that stores ObjectId values is roughly equivalent to sorting by creation time.
IMPORTANT

While ObjectId values should increase over time, they are not necessarily monotonic. This is because they:
- Only contain one second of temporal resolution, so ObjectId values created within the same second do not have a guaranteed ordering, and
- Are generated by clients, which may have differing system clocks.