Auto-generated _id behavior in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the _id field in MongoDB documents?

The _id field uniquely identifies each document in a MongoDB collection. It acts like a primary key to ensure no two documents have the same identifier.

beginner

What happens if you insert a document into MongoDB without specifying an _id field?

MongoDB automatically generates a unique _id of type ObjectId for the document before inserting it.

intermediate

Describe the structure of the default ObjectId generated for the _id field.

The ObjectId is a 12-byte value consisting of: a 4-byte timestamp (seconds since epoch), a 5-byte random value, and a 3-byte incrementing counter. This ensures uniqueness and time ordering.

beginner

Can you insert a document with a custom _id value in MongoDB?

Yes, you can specify your own _id value when inserting a document. MongoDB will use it as the unique identifier instead of generating one.

beginner

What error occurs if you try to insert two documents with the same _id value?

MongoDB throws a duplicate key error because _id must be unique within a collection.

What type does MongoDB use by default for the auto-generated _id field?

A Integer

B String

C UUID

D ObjectId

MongoDB uses ObjectId as the default type for auto-generated _id values.

If you insert a document without an _id, what does MongoDB do?

A Generates a unique <code>_id</code> automatically

B Rejects the insert

C Inserts with a null <code>_id</code>

D Asks the user to provide <code>_id</code>

MongoDB automatically generates a unique _id if none is provided.

Which part of the ObjectId encodes the creation time?

A Last 3 bytes

B First 4 bytes

C Middle 5 bytes

D Entire 12 bytes

The first 4 bytes of ObjectId represent the timestamp of creation.

Can you use a string as a custom _id in MongoDB?

A Only integers are allowed

B No, only ObjectId is allowed

C Yes, any unique value can be used

D Only UUIDs are allowed

MongoDB allows any unique value type as a custom _id, including strings.

What happens if you try to insert two documents with the same _id?

A MongoDB throws a duplicate key error

B The second insert overwrites the first

C MongoDB ignores the second insert

D Both documents are inserted

MongoDB enforces uniqueness on _id and throws an error on duplicates.

Explain how MongoDB generates the _id field automatically when it is not provided.

Think about the parts that make ObjectId unique and time-ordered.

Describe what happens if you insert a document with a custom _id that already exists in the collection.

Consider MongoDB's rule about unique identifiers.