ObjectId and how it is generated in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new ObjectId in MongoDB.

MongoDB

const id = new [1]();

Drag options to blanks, or click blank then click option'

AMongoId

BObject

CIdObject

DObjectId

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Object' instead of 'ObjectId' will not create a MongoDB ID.

Misspelling the constructor name causes errors.

The ObjectId is the correct constructor to create a new unique identifier in MongoDB.

2fill in blank

medium

Complete the code to extract the timestamp from an ObjectId.

MongoDB

const timestamp = id.[1]();

Drag options to blanks, or click blank then click option'

AtoDate

Btimestamp

CgetTimestamp

DgetTime

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'toDate' which is not a valid method.

Confusing 'timestamp' property with a method.

The getTimestamp() method returns the ObjectId's timestamp part as a JavaScript Date object.

3fill in blank

hard

Fix the error in the code to create an ObjectId from a hex string.

MongoDB

const id = new ObjectId([1]);

Drag options to blanks, or click blank then click option'

A"1234567890abcdef12345678"

B'1234567890abcdef12345678'

CObjectId("1234567890abcdef12345678")

D1234567890abcdef12345678

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the hex string without quotes causes a ReferenceError.

Using ObjectId() inside new ObjectId() is incorrect.

The hex string must be passed as a string literal with quotes to the ObjectId constructor.

4fill in blank

hard

Fill both blanks to create an ObjectId and get its timestamp as a Date.

MongoDB

const id = new [1]();
const time = id.[2]();

Drag options to blanks, or click blank then click option'

AObjectId

BtoDate

CgetTimestamp

DIdObject

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong constructor names.

Using non-existent methods like toDate.

Use ObjectId to create the ID and getTimestamp() to get the timestamp as a Date.

5fill in blank

hard

Fill all three blanks to create an ObjectId from a hex string, convert it to a Date, and print it.

MongoDB

const id = new [1]([2]);
const date = id.[3]();
console.log(date);

Drag options to blanks, or click blank then click option'

AObjectId

B"507f1f77bcf86cd799439011"

CtoDate

DgetTimestamp

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the hex string without quotes.

Using 'toDate' which is not a valid method.

Misspelling 'ObjectId' or 'getTimestamp'.

Create the ObjectId with the hex string in quotes, then use getTimestamp() to get the timestamp as a Date.

Practice

1. What does a MongoDB ObjectId primarily represent?

easy

A. A random number generated by the client

B. A unique identifier for documents in a collection

C. A user's login session ID

D. A timestamp of when the database was created

Solution

1. Step 1: Understand the purpose of ObjectId

   ObjectId is designed to uniquely identify each document in a MongoDB collection.

2. Step 2: Differentiate from other identifiers

   It is not a session ID, random number, or database creation timestamp but a unique document ID.

3. Final Answer:

   A unique identifier for documents in a collection -> Option B

4. Quick Check:

   ObjectId = Unique document ID [OK]

Hint: ObjectId is always a unique document ID in MongoDB [OK]

Common Mistakes:

• Thinking ObjectId is a random number
• Confusing ObjectId with session or user IDs
• Assuming it stores database creation time

2. Which of the following is the correct way to create a new ObjectId in MongoDB using the shell?

easy

A. ObjectId()

B. ObjectId.new()

C. new ObjectId()

D. create ObjectId()

Solution

1. Step 1: Recall MongoDB shell syntax

   In the MongoDB shell, new ObjectId() is used to create a new ObjectId instance.

2. Step 2: Check other options for syntax errors

   Options B, C, and D use incorrect syntax or are invalid in MongoDB shell.

3. Final Answer:

   new ObjectId() -> Option C

4. Quick Check:

   Use new ObjectId() to create new IDs [OK]

Hint: Use new ObjectId() in MongoDB shell [OK]

Common Mistakes:

• Using ObjectId() without new keyword
• Trying to call ObjectId as a method
• Using create keyword which is invalid

3. Given the following code snippet in MongoDB shell:

var id = ObjectId();
var timestamp = id.getTimestamp();
print(timestamp);

What does timestamp represent?

medium

A. The creation time of the ObjectId

B. The current system time when print runs

C. The last modified time of the document

D. The expiration time of the ObjectId

Solution

1. Step 1: Understand ObjectId structure

   ObjectId contains a timestamp of when it was created embedded in its first 4 bytes.

2. Step 2: Explain getTimestamp() method

   The getTimestamp() method extracts this creation time from the ObjectId.

3. Final Answer:

   The creation time of the ObjectId -> Option A

4. Quick Check:

   getTimestamp() = ObjectId creation time [OK]

Hint: getTimestamp() returns ObjectId creation time [OK]

Common Mistakes:

• Assuming it returns current time
• Confusing with document modification time
• Thinking it returns expiration time

4. You wrote this code in MongoDB shell:

var id = ObjectId.getTimestamp();

But it throws an error. What is the problem?

medium

A. getTimestamp() is not a static method of ObjectId

B. ObjectId is not defined in MongoDB shell

C. You must pass an argument to getTimestamp()

D. ObjectId.getTimestamp() returns a promise and needs await

Solution

1. Step 1: Check method usage

   getTimestamp() is an instance method, not a static method on ObjectId class.

2. Step 2: Correct usage

   You must create an ObjectId instance first, then call id.getTimestamp().

3. Final Answer:

   getTimestamp() is not a static method of ObjectId -> Option A

4. Quick Check:

   getTimestamp() needs ObjectId instance [OK]

Hint: Call getTimestamp() on ObjectId instance, not class [OK]

Common Mistakes:

• Calling getTimestamp() directly on ObjectId
• Forgetting to create ObjectId instance
• Expecting getTimestamp() to be async

5. You want to generate a MongoDB ObjectId that corresponds to a specific timestamp (e.g., Jan 1, 2020). Which approach is correct?

hard

A. Create an ObjectId with the timestamp bytes set, then fill remaining bytes with zeros

B. Use ObjectId() constructor with a date string argument

C. Manually convert the date to hex and concatenate with random bytes

D. Use ObjectId.createFromTime(timestampInSeconds)

Solution

1. Step 1: Understand ObjectId creation from timestamp

   MongoDB provides ObjectId.createFromTime() to create an ObjectId from a Unix timestamp in seconds.

2. Step 2: Evaluate other options

   Options C and D are manual and error-prone; B is invalid as ObjectId constructor does not accept date strings.

3. Final Answer:

   Use ObjectId.createFromTime(timestampInSeconds) -> Option D

4. Quick Check:

   createFromTime() creates ObjectId from timestamp [OK]

Hint: Use createFromTime() to make ObjectId from timestamp [OK]

Common Mistakes:

• Trying to pass date string to ObjectId()
• Manually building ObjectId hex string
• Ignoring built-in createFromTime() method