What is ObjectId, and when would you not use one as _id?

Short answer

ObjectId is the default unique _id: 12 bytes with a timestamp, randomness, and a counter, unique across clients without a central sequence. Use a natural key instead when the document already has a stable unique identifier you will look up by anyway — an email, an SKU, a UUID from another system.

Why?

_id is immutable and required. Inventing a random string as _id without a uniqueness story is worse than ObjectId. A natural _id avoids a second unique index, but only if the value never changes. Email as _id is a trap the first time someone renames their login.

Example

db.users.insertOne({
  name: "Rahul",
  email: "rahul@example.com"
})
// _id is assigned automatically as an ObjectId

Interview tip

Say _id is immutable. That one sentence prevents a class of 'can I update _id?' follow-ups.

Common mistake

Storing _id as a string in application code and then querying with that string against ObjectId fields.