BSON Types

On this page

• ObjectId
• String
• Timestamps
• Date

BSON is a binary serialization format used to store documents and make remote procedure calls in MongoDB. The BSON specification is located at bsonspec.org.

Each BSON type has both integer and string identifiers as listed in the following table:

You can use these values with the $type operator to query documents by their BSON type. The $type aggregation operator returns the type of an operator expression using one of the listed BSON type strings.

To determine a field’s type, see Check Types in the mongo Shell.

If you convert BSON to JSON, see the Extended JSON reference.

The following sections describe special considerations for particular BSON types.

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.

SEE ALSO

ObjectId()

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. 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. Timestamp values are a 64 bit value where:

• the first 32 bits are a time_t value (seconds since the Unix epoch)
• the second 32 bits are an incrementing ordinal for operations within a given second.

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

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

NOTE

The BSON timestamp type is for internal MongoDB use. For most cases, in application development, you will want to use the BSON date type. See Date for more information.

If you insert a document containing an empty BSON timestamp in a top-level field, the MongoDB server will replace that empty timestamp with the current timestamp value. For example, if you create an insert a document with a timestamp value, as in the following operation:

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var a = new Timestamp();

db.test.insertOne( { ts: a } );

Then, the db.test.find() operation will return a document that resembles the following:

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{ "_id" : ObjectId("542c2b97bac0595474108b48"), "ts" : Timestamp(1412180887, 1) }

If ts were a field in an embedded document, the server would have left it as an empty timestamp value.

Changed in version 2.6: Previously, the server would only replace empty timestamp values in the first two fields, including _id, of an inserted document. Now MongoDB will replace any top-level field.

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.

EXAMPLE

Construct a Date using the new Date() constructor in the mongo shell:

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var mydate1 = new Date()

EXAMPLE

Construct a Date using the ISODate() constructor in the mongo shell:

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var mydate2 = ISODate()

EXAMPLE

Return the Date value as string:

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mydate1.toString()

EXAMPLE

Return the month portion of the Date value; months are zero-indexed, so that January is month 0:

mydate1.getMonth()