Change streams on collections in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to watch changes on the 'users' collection.

MongoDB

const changeStream = db.collection('users').[1]();

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

Aobserve

Bwatch

Clisten

Dsubscribe

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'listen' or 'observe' which are not valid MongoDB methods.

Trying to use 'subscribe' which is not a MongoDB collection method.

The watch() method starts a change stream on the collection.

2fill in blank

medium

Complete the code to specify the pipeline to filter change events for inserts only.

MongoDB

const pipeline = [ { $match: { 'operationType': [1] } } ];

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

A'insert'

B'update'

C'delete'

D'replace'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'update' which watches for modifications, not inserts.

Using 'delete' which watches for removals.

The operationType 'insert' filters events to only new document insertions.

3fill in blank

hard

Fix the error in the code to correctly iterate over change events.

MongoDB

for await (const change of changeStream.[1]()) {
  console.log(change);
}

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

Anext

Biterator

Ccursor

Dstream

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' which is a method on the cursor, not the changeStream itself.

Using 'stream' or 'iterator' which are not valid methods here.

The cursor() method returns an async iterator over the change stream events.

4fill in blank

hard

Fill both blanks to create a change stream with a pipeline filtering for deletes and fullDocument lookup.

MongoDB

const changeStream = db.collection('orders').watch([1], { fullDocument: [2] });

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

A[{ $match: { operationType: 'delete' } }]

Btrue

Cfalse

D[{ $match: { operationType: 'insert' } }]

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong operationType in the pipeline.

Setting fullDocument to false which disables full document lookup.

The pipeline filters for 'delete' operations, and fullDocument: true returns the full document on change.

5fill in blank

hard

Fill all three blanks to handle change events and close the stream on error.

MongoDB

try {
  for await (const change of changeStream.[1]()) {
    console.log(change.[2]);
  }
} catch (error) {
  console.error(error);
  changeStream.[3]();
}

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

Acursor

BfullDocument

Cclose

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' instead of 'cursor()' for iteration.

Accessing 'document' instead of 'fullDocument'.

Not closing the stream on error.

Use cursor() to iterate, access fullDocument for changed data, and close() to stop the stream on error.

Practice

1. What is the main purpose of using change streams on a MongoDB collection?

easy

A. To create indexes on the collection

B. To backup the collection data automatically

C. To watch for real-time changes happening in the collection

D. To delete documents based on a condition

Solution

1. Step 1: Understand what change streams do

   Change streams allow applications to listen for real-time changes like inserts, updates, or deletes in a collection.

2. Step 2: Compare options with this purpose

   Only To watch for real-time changes happening in the collection describes watching for real-time changes, which matches the purpose of change streams.

3. Final Answer:

   To watch for real-time changes happening in the collection -> Option C

4. Quick Check:

   Change streams = watch real-time changes [OK]

Hint: Change streams = real-time watch on collection changes [OK]

Common Mistakes:

• Thinking change streams backup data
• Confusing change streams with indexing
• Assuming change streams delete documents

2. Which of the following is the correct way to start a change stream on a collection named orders in MongoDB?

easy

A. db.orders.changeStream()

B. db.orders.watch()

C. db.orders.listen()

D. db.orders.observe()

Solution

1. Step 1: Recall the MongoDB method for change streams

   The correct method to open a change stream on a collection is watch().

2. Step 2: Match the method with the collection name

   Using db.orders.watch() correctly starts the change stream on the orders collection.

3. Final Answer:

   db.orders.watch() -> Option B

4. Quick Check:

   Change stream method = watch() [OK]

Hint: Use .watch() to start change streams on collections [OK]

Common Mistakes:

• Using non-existent methods like changeStream()
• Confusing with listen() or observe() which don't exist
• Misspelling the method name

3. Given the following code snippet, what will be the output when a new document is inserted into the products collection?

const changeStream = db.products.watch();
changeStream.on('change', (change) => {
  console.log(change.operationType);
});

medium

A. 'insert'

B. 'update'

C. 'delete'

D. 'replace'

Solution

1. Step 1: Understand the event listener on change stream

   The code listens for any change and logs the operationType of that change.

2. Step 2: Identify the operation type for inserting a document

   When a new document is inserted, the operationType is insert.

3. Final Answer:

   'insert' -> Option A

4. Quick Check:

   Insert operation logs 'insert' [OK]

Hint: Insert triggers operationType 'insert' in change stream [OK]

Common Mistakes:

• Confusing insert with update or delete
• Expecting full document instead of operationType
• Misreading event listener output

4. You wrote this code to watch changes on the users collection but it throws an error:

const changeStream = db.users.watch({ operationType: 'insert' });

What is the mistake?

medium

A. The operationType should be capitalized as 'Insert'

B. The collection name should be user not users

C. You must call start() after watch()

D. The watch method does not accept a filter object directly

Solution

1. Step 1: Check the argument to watch()

   The watch() method expects an array pipeline for filtering, not a direct object.

2. Step 2: Understand correct filtering syntax

   To filter by operationType, you must pass a pipeline like [{ $match: { operationType: 'insert' } }].

3. Final Answer:

   The watch method does not accept a filter object directly -> Option D

4. Quick Check:

   Filter must be pipeline array, not object [OK]

Hint: Use pipeline array for filters in watch() [OK]

Common Mistakes:

• Passing filter as object instead of pipeline array
• Wrong collection name assumption
• Expecting a start() method
• Capitalizing operationType values incorrectly

5. You want to build a live notification system that only alerts when a document in the tasks collection is updated and the status field changes to 'completed'. Which change stream pipeline correctly filters these changes?

hard

A. [{ $match: { operationType: 'update', 'updateDescription.updatedFields.status': 'completed' } }]

B. [{ $match: { operationType: 'insert', 'fullDocument.status': 'completed' } }]

C. [{ $match: { 'updateDescription.updatedFields.status': 'completed' } }]

D. [{ $match: { operationType: 'delete', 'updateDescription.updatedFields.status': 'completed' } }]

Solution

1. Step 1: Identify the operation type needed

   We want to watch only updates, so operationType must be 'update'.

2. Step 2: Filter for status field changed to 'completed'

   The updateDescription.updatedFields.status contains changed fields; we check if it equals 'completed'.

3. Step 3: Combine filters in a pipeline

   The correct pipeline is [{ $match: { operationType: 'update', 'updateDescription.updatedFields.status': 'completed' } }].

4. Final Answer:

   [{ $match: { operationType: 'update', 'updateDescription.updatedFields.status': 'completed' } }] -> Option A

5. Quick Check:

   Filter updates with status='completed' = [{ $match: { operationType: 'update', 'updateDescription.updatedFields.status': 'completed' } }] [OK]

Hint: Filter update and updatedFields.status in $match pipeline [OK]

Common Mistakes:

• Filtering inserts or deletes instead of updates
• Checking fullDocument instead of updateDescription
• Missing operationType filter
• Using wrong field path in filter