Its very complex combination of Objectids makes all the _id fields unique. We have option to into own value into _id field.

Object ID= [4 bytes seconds since epoch, 3 bytes machine hash, 2 bytes process ID, 3 bytes counter]

An ObjectId is a 12-byte BSON type having the following structure:

•    The first 4 bytes representing the seconds since the unix epoch
•    The next 3 bytes are the machine identifier
•    The next 2 bytes consists of process id
•    The last 3 bytes are a random counter value

Benefit of ObjectIDs

• ObjectId make document unique
• ObjectID store timestamp .we Can easily get creation time of ObjectIds by using getTimeStamp() function. So, no more need to column like createDate or insertedDate
• Index automatically created into _id field for improve index. It’s helpful in sorting.

How Objectids generate in static increment nature.

Creating New ObjectId

To generate a new ObjectId use the following code:

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

Insert own unique key

We can insert own unique value for _id.

db.foo.insert({_id: ‘abc’})

Creation Timestamp of a Document

Objectid store 4 byte tmestamp. You can fetch the creation time of a document using getTimeStamp function :

 when you will run command on console. This will return the creation time of this document in ISO Date format:

ISODate(“2015-05-27T09:00:17Z”)

Convert ObjectId to String

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

x.str (x is object here)Or
ObjectId(“556587a1461650e970aec7a0”).str
this command will return the string format of object like this

556587a1461650e970aec7a0