Auto-generated _id behavior in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the type of the default _id field in MongoDB?

When you insert a document into a MongoDB collection without specifying the _id field, MongoDB automatically generates one. What is the data type of this auto-generated _id?

A An ObjectId, a 12-byte BSON type

B A string representing a UUID

C An integer starting from 1 and incrementing

D A timestamp value only

💡 Hint

Think about the unique identifier MongoDB uses internally for documents.

MongoDB generates an ObjectId by default for the _id field. It is a 12-byte BSON type that includes a timestamp, machine identifier, process id, and a counter to ensure uniqueness.

❓ query_result

intermediate

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

Consider inserting two documents into a MongoDB collection where both documents have the same _id value explicitly set. What will happen?

A Both documents will be inserted successfully with duplicate _id values.

B The second insert will overwrite the first document silently.

C MongoDB will automatically generate a new _id for the second document.

D The second insert will fail with a duplicate key error.

💡 Hint

Remember that _id must be unique in a collection.

The _id field is a unique identifier for documents in a collection. Attempting to insert a document with an _id that already exists causes a duplicate key error.

🧠 Conceptual

advanced

Why does MongoDB use a 12-byte ObjectId for _id by default?

MongoDB's default _id is a 12-byte ObjectId. Which of the following best explains the reason for this design?

A To store a simple incrementing integer for easy sorting

B To embed creation time and ensure uniqueness across machines and processes

C To save space by using a fixed 4-byte integer

D To allow storing large binary data inside the _id field

💡 Hint

Think about what information is useful for a unique identifier in distributed systems.

The 12-byte ObjectId includes a timestamp, machine identifier, process id, and counter. This design ensures uniqueness and encodes creation time for sorting.

📝 Syntax

advanced

Which MongoDB insert command will NOT generate an _id automatically?

Given the following insert commands, which one will NOT cause MongoDB to generate an _id field automatically?

MongoDB

db.collection.insertOne({ name: "Alice", _id: "custom_id_123" })

A db.collection.insertOne({ name: "Alice" })

B db.collection.insertOne({ name: "Alice", _id: ObjectId() })

C db.collection.insertOne({ name: "Alice", _id: "custom_id_123" })

D db.collection.insertOne({ name: "Alice", _id: null })

💡 Hint

When you provide an _id explicitly, MongoDB does not generate one.

If you specify the _id field explicitly in the document, MongoDB uses that value and does not generate a new ObjectId.

🔧 Debug

expert

Why does this MongoDB insert fail with a duplicate key error?

You run the following commands: 1. db.users.insertOne({ name: "Bob" }) 2. db.users.insertOne({ _id: ObjectId("507f1f77bcf86cd799439011"), name: "Carol" }) 3. db.users.insertOne({ _id: ObjectId("507f1f77bcf86cd799439011"), name: "Dave" }) The third insert fails with a duplicate key error. Why?

A Because the _id value in the third insert is the same as the second insert's _id

B Because the first insert generated an _id that conflicts with the third insert

C Because ObjectId values cannot be used as _id fields

D Because the collection does not allow inserting documents with names starting with 'D'

💡 Hint

Check the _id values used in the inserts carefully.

The third insert uses the same explicit _id as the second insert, causing a duplicate key error because _id must be unique.