ObjectId and how it is generated in MongoDB - Mini Project: Build & Apply

Understanding ObjectId and How It Is Generated in MongoDB

📖 Scenario: You are working on a simple MongoDB database for a small online store. Each product needs a unique identifier that MongoDB automatically generates. This identifier is called ObjectId. You want to learn how to create documents with ObjectId and understand its structure.

🎯 Goal: Build a MongoDB collection with documents that use ObjectId as their unique identifier. Learn how ObjectId is generated and how to access its components.

📋 What You'll Learn

Create a MongoDB collection named products with documents containing _id as ObjectId

Insert a document with specific fields: name and price

Extract and display the timestamp part of the ObjectId

Understand the components of ObjectId by accessing its properties

💡 Why This Matters

🌍 Real World

ObjectId is used in MongoDB to uniquely identify documents and embed creation time information, which helps in tracking when data was added.

💼 Career

Understanding ObjectId is essential for database developers and administrators working with MongoDB to manage data efficiently and perform time-based queries.

1

Create a products collection and insert a document with ObjectId

Create a MongoDB collection called products and insert one document with the fields name set to 'Laptop' and price set to 1200. Do not specify the _id field; MongoDB will generate an ObjectId automatically.

MongoDB

// Insert a document into the products collection
// Your code here

Hint

Use insertOne on db.products with the document containing name and price.

2

Retrieve the inserted document and store its _id in a variable

Write a query to find the document where name is 'Laptop' and store its _id field in a variable called productId.

MongoDB

db.products.insertOne({ name: 'Laptop', price: 1200 })
// Find the document with name 'Laptop' and store its _id in productId
// Your code here

Hint

Use findOne to get the document and then access its _id property.

3

Extract the timestamp from the ObjectId

Use the getTimestamp() method on the productId variable to get the creation time of the ObjectId and store it in a variable called creationTime.

MongoDB

db.products.insertOne({ name: 'Laptop', price: 1200 })
const product = db.products.findOne({ name: 'Laptop' })
const productId = product._id
// Get the timestamp from productId and store in creationTime
// Your code here

Hint

Call getTimestamp() on productId to get the creation date.

4

Access the ObjectId components and complete the document with a new field

Add a new field called createdAt to the products collection document with the value of creationTime. Use updateOne with the _id equal to productId to set this field.

MongoDB

db.products.insertOne({ name: 'Laptop', price: 1200 })
const product = db.products.findOne({ name: 'Laptop' })
const productId = product._id
const creationTime = productId.getTimestamp()
// Update the document to add createdAt field with creationTime
// Your code here

Hint

Use updateOne with a filter on _id and the $set operator to add the createdAt field.

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