ObjectId is a 12-byte BSON type, constructed using:
In MongoDB, documents stored in a collection require a unique _id field that acts as a primary key. Because ObjectIds are small, most likely unique, and fast to generate, MongoDB uses ObjectIds as the default value for the _id field if the _id field is not specified; i.e., the mongod adds the _id field and generates a unique ObjectId to assign as its value.
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 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, not the mongod process.
Also consider the BSON Documents section for related information on MongoDB’s document orientation.
The mongo shell provides the ObjectId() wrapper class to generate a new ObjectId, and to provide the following helper attribute and methods:
str
The hexadecimal string value of the ObjectId() object.
Returns the timestamp portion of the ObjectId() object as a Date.
Returns the string representation of the ObjectId() object. The returned string literal has the format “ObjectId(...)”.
Changed in version 2.2: In previous versions ObjectId.toString() returns the value of the ObjectId as a hexadecimal string.
Returns the value of the ObjectId() object as a hexadecimal string. The returned string is the str attribute.
Changed in version 2.2: In previous versions ObjectId.valueOf() returns the ObjectId() object.
Consider the following uses ObjectId() class in the mongo shell:
To generate a new ObjectId, use the ObjectId() constructor with no argument:
x = ObjectId()
In this example, the value of x would be:
ObjectId("507f1f77bcf86cd799439011")
To generate a new ObjectId using the ObjectId() constructor with a unique hexadecimal string:
y = ObjectId("507f191e810c19729de860ea")
In this example, the value of y would be:
ObjectId("507f191e810c19729de860ea")
To return the timestamp of an ObjectId() object, use the getTimestamp() method as follows:
ObjectId("507f191e810c19729de860ea").getTimestamp()
This operation will return the following Date object:
ISODate("2012-10-17T20:46:22Z")
Access the str attribute of an ObjectId() object, as follows:
ObjectId("507f191e810c19729de860ea").str
This operation will return the following hexadecimal string:
507f191e810c19729de860ea
To return the string representation of an ObjectId() object, use the toString() method as follows:
ObjectId("507f191e810c19729de860ea").toString()
This operation will return the following output:
ObjectId("507f191e810c19729de860ea")
To return the value of an ObjectId() object as a hexadecimal string, use the valueOf() method as follows:
ObjectId("507f191e810c19729de860ea").valueOf()
This operation returns the following output:
507f191e810c19729de860ea