BSON TypesBSON类型¶

ObjectId¶

ObjectIds are small, likely unique, fast to generate, and ordered.Objectid很小，可能是唯一的，生成速度很快，而且是有序的。ObjectId values are 12 bytes in length, consisting of:ObjectId值的长度为12字节，包括：

• a 4-byte timestamp value, representing the ObjectId’s creation, measured in seconds since the Unix epoch一个4字节的时间戳值，表示ObjectId的创建，从Unix时代开始以秒为单位
• a 5-byte random value一个5字节的随机值
• a 3-byte incrementing counter, initialized to a random value初始化为随机值的3字节递增计数器

While the BSON format itself is little-endian, the timestamp and counter values are big-endian, with the most significant bytes appearing first in the byte sequence.虽然BSON格式本身是小端，但时间戳和计数器值是大端，最重要的字节首先出现在字节序列中。

In MongoDB, each document stored in a collection requires a unique _id field that acts as a primary key.在MongoDB中，存储在集合中的每个文档都需要一个作为主键的唯一_id字段。If an inserted document omits the _id field, the MongoDB driver automatically generates an ObjectId for the _id field.如果插入的文档省略了_id字段，MongoDB驱动程序会自动为_id字段生成ObjectId。

This also applies to documents inserted through update operations with upsert: true.这也适用于通过upsert: true的更新操作插入的文档。

MongoDB clients should add an _id field with a unique ObjectId.MongoDB客户端应该添加一个带有唯一ObjectId的_id字段。 Using ObjectIds for the _id field provides the following additional benefits:将objectID用于_id字段可提供以下附加好处：

• in the mongo shell, you can access the creation time of the ObjectId, using the ObjectId.getTimestamp() method.在mongoshell中，可以使用ObjectId.getTimestamp()方法访问ObjectId的创建时间。
• sorting on an _id field that stores ObjectId values is roughly equivalent to sorting by creation time.在存储ObjectId值的_id字段上排序大致相当于按创建时间排序。

  Important

  While ObjectId values should increase over time, they are not necessarily monotonic.虽然ObjectId值应该随着时间的推移而增加，但它们不一定是单调的。This is because they:这是因为它们：

  • Only contain one second of temporal resolution, so ObjectId values created within the same second do not have a guaranteed ordering, and只包含一秒钟的时间分辨率，因此在同一秒钟内创建的ObjectId值没有保证的顺序，并且
  • Are generated by clients, which may have differing system clocks.由客户端生成，这些客户端可能具有不同的系统时钟。

String字符串¶

BSON strings are UTF-8.BSON字符串是UTF-8。In general, drivers for each programming language convert from the language’s string format to UTF-8 when serializing and deserializing BSON.通常，在序列化和反序列化BSON时，每种编程语言的驱动程序都会从该语言的字符串格式转换为UTF-8。This makes it possible to store most international characters in BSON strings with ease.这使得在BSON字符串中轻松存储大多数国际字符成为可能。[1] In addition, MongoDB $regex queries support UTF-8 in the regex string.此外，MongoDB $regex查询在regex字符串中支持UTF-8。

Timestamps时间戳¶

BSON has a special timestamp type for internal MongoDB use and is not associated with the regular Date type.BSON有一个特殊的时间戳类型供内部MongoDB使用，并且与常规日期类型没有关联。This internal timestamp type is a 64 bit value where:此内部时间戳类型是64位值，其中：

• the most significant 32 bits are a time_t value (seconds since the Unix epoch)最高有效的32位是time_t值（自Unix纪元起的秒数）
• the least significant 32 bits are an incrementing ordinal for operations within a given second.最低有效的32位是给定秒内操作的ordinal。

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.虽然BSON格式本身是小端，因此首先存储最低有效位，但mongod实例总是在所有平台上比较time_t值，然后比较ordinal值，而不管字节序是多少。

Within a single mongod instance, timestamp values are always unique.在单个mongod实例中，时间戳值总是唯一的。

In replication, the oplog has a ts field.在副本中，oplog有一个ts字段。The values in this field reflect the operation time, which uses a BSON timestamp value.此字段中的值反映操作时间，它使用BSON时间戳值。

When inserting a document that contains top-level fields with empty timestamp values, MongoDB replaces the empty timestamp values with the current timestamp value, with the following exception.在插入包含具有空时间戳值的顶级字段的文档时，MongoDB将用当前时间戳值替换空时间戳值，但以下情况除外。If the _id field itself contains an empty timestamp value, it will always be inserted as is and not replaced.如果_id字段本身包含空的时间戳值，则它将始终按原样插入而不被替换。

db.test.insertOne( { ts: new Timestamp() } );

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

Date¶

BSON Date is a 64-bit integer that represents the number of milliseconds since the Unix epoch (Jan 1, 1970).BSON Date是一个64位整数，表示自Unix epoch（1970年1月1日）以来的毫秒数。This results in a representable date range of about 290 million years into the past and future.这产生了过去和未来约2.9亿年的可代表日期范围。

The official BSON specification refers to the BSON Date type as the UTC datetime.正式的BSON规范将BSON日期类型称为UTC datetime。

BSON Date type is signed.BSON日期类型是有符号的。[2] Negative values represent dates before 1970.负值表示1970年之前的日期。

var mydate1 = new Date()

var mydate2 = ISODate()

mydate1.toString()

mydate1.getMonth()