# The MongoDB\BSON\ObjectId class

(mongodb >=1.0.0)

## はじめに

BSON type for an » ObjectId. The value consists of 12 bytes, where the first four bytes are a timestamp that reflect the ObjectId's creation. Specifically, the value consists of:

- 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 driver automatically generates an ObjectId for the _id field.

Using ObjectIds for the _id field provides the following additional benefits:

- The creation time of the ObjectId may be accessed using the MongoDB\BSON\ObjectId::getTimestamp() method.
- Sorting on an _id field that stores ObjectId values is roughly equivalent to sorting by creation time.

## クラス概要

final MongoDB\BSON\ObjectId implements MongoDB\BSON\ObjectIdInterface , MongoDB\BSON\Type , Serializable , JsonSerializable {
/* メソッド */
final public __construct ([ string `$id` ] )
final public getTimestamp ( void ) : int
final public jsonSerialize ( void ) : mixed
final public serialize ( void ) : string
final public __toString ( void ) : string
final public unserialize ( string `$serialized` ) : void
}

## 変更履歴

バージョン | 説明
--- | ---
1.3.0 | Renamed from MongoDB\BSON\ObjectID to MongoDB\BSON\ObjectId.
1.2.0 | Implements Serializable and JsonSerializable.

## 目次

- MongoDB\BSON\ObjectId::__construct — Construct a new ObjectId
- MongoDB\BSON\ObjectId::getTimestamp — Returns the timestamp component of this ObjectId
- MongoDB\BSON\ObjectId::jsonSerialize — Returns a representation that can be converted to JSON
- MongoDB\BSON\ObjectId::serialize — Serialize an ObjectId
- MongoDB\BSON\ObjectId::__toString — Returns the hexidecimal representation of this ObjectId
- MongoDB\BSON\ObjectId::unserialize — Unserialize an ObjectId