BSON Types¶

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Comparison/Sort Order
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.

BSON supports the following data types as values in documents. Each data type has a corresponding number and string alias that can be used with the $type operator to query documents by BSON type.

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

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

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

Comparison/Sort Order¶

When comparing values of different BSON types, MongoDB uses the following comparison order, from lowest to highest:

MinKey (internal type)
Null
Numbers (ints, longs, doubles)
Symbol, String
Object
Array
BinData
ObjectId
Boolean
Date
Timestamp
Regular Expression
MaxKey (internal type)

MongoDB treats some types as equivalent for comparison purposes. For instance, numeric types undergo conversion before comparison.

Changed in version 3.0.0: Date objects sort before Timestamp objects. Previously Date and Timestamp objects sorted together.

The comparison treats a non-existent field as it would an empty BSON Object. As such, a sort on the a field in documents { } and { a: null } would treat the documents as equivalent in sort order.

With arrays, a less-than comparison or an ascending sort compares the smallest element of arrays, and a greater-than comparison or a descending sort compares the largest element of the arrays. As such, when comparing a field whose value is a single-element array (e.g. [ 1 ]) with non-array fields (e.g. 2), the comparison is between 1 and 2. A comparison of an empty array (e.g. [ ]) treats the empty array as less than null or a missing field.

MongoDB sorts BinData in the following order:

First, the length or size of the data.
Then, by the BSON one-byte subtype.
Finally, by the data, performing a byte-by-byte comparison.

The following sections describe special considerations for particular BSON types.

ObjectId¶

ObjectIds are: small, likely unique, fast to generate, and ordered. These values consists of 12-bytes, where the first four bytes are a timestamp that reflect the ObjectId’s creation. Refer to the ObjectId documentation for more information.

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.insert( { 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.

Changed in version 2.0: 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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