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MongoDB uses ObjectIds as the default value of _id field of each document, which is generated while the creation of any document. The complex combination of ObjectId makes all the _id fields unique.

Creating New ObjectId

To generate a new ObjectId use the following code −

>newObjectId = ObjectId()

The above statement returned the following uniquely generated id −

ObjectId("5349b4ddd2781d08c09890f3")

Instead of MongoDB generating the ObjectId, you can also provide a 12-byte id −

>myObjectId = ObjectId("5349b4ddd2781d08c09890f4")

Creating Timestamp of a Document

Since the _id ObjectId by default stores the 4-byte timestamp, in most cases you do not need to store the creation time of any document. You can fetch the creation time of a document using getTimestamp method −

>ObjectId("5349b4ddd2781d08c09890f4").getTimestamp()

This will return the creation time of this document in ISO date format −

ISODate("2014-04-12T21:49:17Z")

Converting ObjectId to String

In some cases, you may need the value of ObjectId in a string format. To convert the ObjectId in string, use the following code −

>newObjectId.str

The above code will return the string format of the Guid −

5349b4ddd2781d08c09890f3