MongoDB ObjectId

In the previous chapters, we have already used MongoDB's ObjectId.

In this chapter, we will learn about the structure of ObjectId.

ObjectId is a 12-byte BSON type data, with the following format:

  • The first 4 bytes represent the timestamp
  • The next 3 bytes are the machine identifier code
  • The next two bytes consist of the process ID (PID)
  • The last three bytes are a random number.

Documents stored in MongoDB must have an "_id" key. The value of this key can be of any type, and by default it is an ObjectId object.

Within a collection, each document has a unique "_id" value, which ensures that every document in the collection can be uniquely identified.

The main reason MongoDB uses ObjectId instead of other conventional practices (such as auto-incrementing primary keys) is because synchronizing auto-incrementing primary key values across multiple servers is both laborious and time-consuming.

Create a new ObjectId

Use the following code to generate a new ObjectId:

>newObjectId = ObjectId()

The above statement returns the following uniquely generated id:

ObjectId("5349b4ddd2781d08c09890f3")

You can also use the generated id to replace the ObjectId automatically generated by MongoDB:

>myObjectId = ObjectId("5349b4ddd2781d08c09890f4")

Timestamp of document creation

Since ObjectId stores a 4-byte timestamp, you don't need to save a timestamp field for your documents. You can use the getTimestamp function to obtain the creation time of the document:

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

The above code will return the document creation time in ISO format:

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

Convert ObjectId to string

In some cases, you may need to convert ObjectId to string format. You can use the following code:

>new ObjectId().str

The above code will return a string in Guid format:

5349b4ddd2781d08c09890f3
Other Extensions