ObjectId and _id

Every document needs a unique _id inside its collection. If you do not set one on insert, MongoDB generates an ObjectId. That is the default, and it is the right default for almost every collection.

{
  _id: ObjectId("64a1b2c3d4e5f6a7b8c90001"),
  name: "Rahul",
  city: "Bangalore"
}

What an ObjectId is

A 12-byte value, usually shown as 24 hex characters. At a high level it carries a timestamp, some randomness, and a counter. You can roughly tell when it was created. You do not need a central ID service. You do not need to remember the byte layout in an interview — timestamp plus uniqueness is enough.

Querying by _id

db.users.find({
  _id: ObjectId("64a1b2c3d4e5f6a7b8c90001")
})

{ _id: ObjectId("64a1b2c3d4e5f6a7b8c90001"), name: "Rahul", city: "Bangalore", ... }

// Matches nothing. The field is not a string.
db.users.find({
  _id: "64a1b2c3d4e5f6a7b8c90001"
})

In Node.js the driver gives you ObjectId from the mongodb package. Wrap the string from the URL before you query. In the shell, ObjectId("...") does the same.

import { ObjectId } from "mongodb";

await users.findOne({
  _id: new ObjectId(req.params.id)
});

You can set _id to a string (an email, a slug). Do that only when the value is already unique and stable and you will query it as a string everywhere. Mixing ObjectId documents and string _ids in one collection is how you get 'document not found' bugs.