ObjectId and how it is generated in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What does the first 4 bytes of a MongoDB ObjectId represent?

In MongoDB, an ObjectId is a 12-byte unique identifier. What information is stored in the first 4 bytes of this ObjectId?

A The timestamp when the ObjectId was generated, in seconds since the Unix epoch

B A random number generated by the client

C The machine identifier where the ObjectId was created

D The process ID of the client generating the ObjectId

💡 Hint

Think about what helps to sort ObjectIds chronologically.

The first 4 bytes of a MongoDB ObjectId store the timestamp in seconds since the Unix epoch. This allows ObjectIds to be roughly sorted by creation time.

❓ query_result

intermediate

What is the length of a MongoDB ObjectId in bytes?

You retrieve an ObjectId from a MongoDB document. How many bytes does this ObjectId contain?

A 8 bytes

B 16 bytes

C 12 bytes

D 24 bytes

💡 Hint

Remember the fixed size of ObjectId in MongoDB.

A MongoDB ObjectId is always 12 bytes long, consisting of timestamp, machine id, process id, and counter.

📝 Syntax

advanced

Which code snippet correctly creates a new ObjectId in MongoDB shell?

Select the valid MongoDB shell command to generate a new ObjectId.

A new ObjectId

B ObjectId()

C ObjectId.new()

D new ObjectId()

💡 Hint

In JavaScript, how do you create a new instance of an object?

In MongoDB shell (JavaScript), you create a new ObjectId by calling new ObjectId() with parentheses.

🔧 Debug

advanced

Why does this ObjectId creation code fail? db.collection.insertOne({_id: ObjectId})

You run this command in MongoDB shell: db.collection.insertOne({_id: ObjectId}). It fails. Why?

A ObjectId is a function and must be called with parentheses to create an instance

B ObjectId is not defined in MongoDB shell

C You cannot specify _id manually in insertOne

D The collection name is missing

💡 Hint

Think about how to create a new ObjectId value.

ObjectId is a function constructor. You must call it with parentheses like ObjectId() to create a new ObjectId value.

🧠 Conceptual

expert

Which part of the ObjectId ensures uniqueness across different machines?

MongoDB ObjectId includes several parts. Which part helps ensure that ObjectIds generated on different machines are unique?

A The 4-byte timestamp

B The 3-byte machine identifier derived from the host's network interface

C The 2-byte process id

D The 3-byte incrementing counter

💡 Hint

Consider what differentiates machines in a network.

The 3-byte machine identifier is generated from the machine's network interface and helps ensure uniqueness across different machines.