BSON Types

ObjectId

ObjectIds are small, likely unique, fast to generate, and ordered. ObjectId values are 12 bytes in length, consisting of:

• A 4-byte timestamp, representing the ObjectId's creation, measured in seconds since the Unix epoch.

• A 5-byte random value generated once per process. This random value is unique to the machine and process.

• A 3-byte incrementing counter, initialized to a random value.

For timestamp and counter values, the most significant bytes appear first in the byte sequence (big-endian). This is unlike other BSON values, where the least significant bytes appear first (little-endian).

If an integer value is used to create an ObjectId, the integer replaces the timestamp.

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:

• You can access ObjectId creation time in mongosh using the ObjectId.getTimestamp() method.

• ObjectIds are approximately ordered by creation time, but are not perfectly ordered. Sorting a collection on an _id field containing 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.

Use the ObjectId() methods to set and retrieve ObjectId values.

Starting in MongoDB 5.0, mongosh replaces the legacy mongo shell. The ObjectId() methods work differently in mongosh than in the legacy mongo shell. For more information on the legacy methods, see Legacy mongo Shell.

String

BSON strings are UTF-8. In general, drivers for each programming language convert from the language's string format to UTF-8 when serializing and deserializing BSON. This makes it possible to store most international characters in BSON strings with ease. [1] In addition, MongoDB $regex queries support UTF-8 in the regex string.

Timestamps

BSON has a special timestamp type for internal MongoDB use and is not associated with the regular Date type. This internal timestamp type is a 64 bit value where:

• the most significant 32 bits are a time_t value (seconds since the Unix epoch)

• the least significant 32 bits are an incrementing ordinal for operations within a given second.

While the BSON format is little-endian, and therefore stores the least significant bits first, the mongod instance always compares the time_t value before the ordinal value on all platforms, regardless of endianness.

In replication, the oplog has a ts field. The values in this field reflect the operation time, which uses a BSON timestamp value.

Within a single mongod instance, timestamp values in the oplog are always unique.

Date

BSON Date is a 64-bit integer that represents the number of milliseconds since the Unix epoch (Jan 1, 1970). This results in a representable date range of about 290 million years into the past and future.

The official BSON specification refers to the BSON Date type as the UTC datetime.

BSON Date type is signed. [2] Negative values represent dates before 1970.

var mydate1 = new Date()

var mydate2 = ISODate()

mydate1.toString()

mydate1.getMonth()