Every document in a MongoDB collection must have an _id that contains a unique value. This field acts as a primary key to these documents. The primary keys are used to uniquely identify the documents, and they are always indexed.

The following is an example of a sample document inside a MongoDB collection:

_id:ObjectId("61a30de3e385457dc19e6b54")
name:"MacBook Pro"
category:"Electronics"
__v:0

In a MongoDB collection, if you insert a document without an _id field, the MongoDB driver will autogenerate a unique ID and add it to the document.

So, when you retrieve the inserted document, you will find _id is generated with a unique value of random text. When the _id field is automatically added by the driver, the value is generated using ObjectId.

The ObjectId value is designed to generate lightweight code that is unique across different machines. It generates a unique value of 12 bytes, where the first 4 bytes represent the timestamp, bytes 5 to 9 represent a random value, and the last 3 bytes are an incremental counter.

MongoDB supports a technique called sharding, where a dataset is distributed and stored on different machines. When a collection is sharded, its documents are physically located on different machines. Even so, ObjectId can ensure that the values will be unique in the collection across different machines.

Check out some of my other blog posts related to MongoDB:

Learn to delete documents in MongoDB in my blog post here.

Learn to replace documents in MongoDB in my blog post here.

Learn to update documents in MongoDB in my blog post here.

Learn to sort documents in MongoDB in my blog post here.

Learn to skip & limit documents in MongoDB in my blog post here.