BSON Types¶

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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:

Type	Number	Alias	Notes
Double	1	“double”
String	2	“string”
Object	3	“object”
Array	4	“array”
Binary data	5	“binData”
Undefined	6	“undefined”	Deprecated.
ObjectId	7	“objectId”
Boolean	8	“bool”
Date	9	“date”
Null	10	“null”
Regular Expression	11	“regex”
DBPointer	12	“dbPointer”	Deprecated.
JavaScript	13	“javascript”
Symbol	14	“symbol”	Deprecated.
JavaScript (with scope)	15	“javascriptWithScope”
32-bit integer	16	“int”
Timestamp	17	“timestamp”
64-bit integer	18	“long”
Decimal128	19	“decimal”	New in version 3.4.
Min key	-1	“minKey”
Max key	127	“maxKey”

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 3-byte machine identifier,
a 2-byte process id, 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

The relationship between the order of ObjectId values and generation time is not strict within a single second. If multiple systems, or multiple processes or threads on a single system generate values, within a single second; ObjectId values do not represent a strict insertion order. Clock skew between clients can also result in non-strict ordering even for values because client drivers generate ObjectId values.

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.

[1]	Given strings using UTF-8 character sets, using `sort()` on strings will be reasonably correct. However, because internally `sort()` uses the C++ `strcmp` api, the sort order may handle some characters incorrectly.

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:

var a = new Timestamp();

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

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

{ "_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:

var mydate1 = new Date()

Example

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

var mydate2 = ISODate()

Example

Return the Date value as string:

mydate1.toString()

Example

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

mydate1.getMonth()

[2] Prior to version 2.0, Date values were incorrectly interpreted as unsigned integers, which affected sorts, range queries, and indexes on Date fields. Because indexes are not recreated when upgrading, please re-index if you created an index on Date values with an earlier version, and dates before 1970 are relevant to your application.

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