Change stream events (insert, update, delete) in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to watch for insert events in a MongoDB collection.

MongoDB

const changeStream = collection.watch([{ $match: { operationType: [1] } }]);

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

A"insert"

B"delete"

C"update"

D"replace"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'update' instead of 'insert' will watch for update events, not inserts.

Forgetting quotes around the operationType value causes syntax errors.

The operationType filter should be set to "insert" to watch for insert events.

2fill in blank

medium

Complete the code to listen for delete events from the change stream.

MongoDB

changeStream.on('change', (change) => { if (change.operationType === [1]) { console.log('Document deleted'); } });

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

A"insert"

B"update"

C"replace"

D"delete"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'update' or 'insert' will not detect delete events.

Missing quotes around the string causes errors.

To detect delete events, check if operationType equals "delete".

3fill in blank

hard

Fix the error in the code to correctly filter for update events in the change stream pipeline.

MongoDB

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

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

A"update"

Bupdate

Cupdates

D"updates"

Attempts:

3 left

💡 Hint

Common Mistakes

Using unquoted update causes syntax errors.

Using plural 'updates' is incorrect; the correct value is 'update'.

The operationType value must be a string with quotes, so use "update".

4fill in blank

hard

Fill both blanks to create a change stream that listens for insert and delete events.

MongoDB

const pipeline = [{ $match: { operationType: { $in: [[1], [2]] } } }];

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 unquoted strings causes errors.

Including wrong operation types like 'update' or 'replace' changes the filter.

The $in operator filters for multiple operation types. Use "insert" and "delete" to watch for those events.

5fill in blank

hard

Fill all three blanks to log the document key, operation type, and full document on insert events.

MongoDB

changeStream.on('change', (change) => { if (change.operationType === [1]) { console.log('Key:', change.[2]); console.log('Full Document:', change.[3]); } });

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

A"insert"

BdocumentKey

CfullDocument

Dns

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'update' or 'delete' instead of 'insert' changes the event type.

Accessing 'ns' instead of 'documentKey' or 'fullDocument' gives namespace info, not document data.

For insert events, operationType is "insert". The document key is in documentKey, and the full inserted document is in fullDocument.

Practice

1. What does a MongoDB change stream primarily allow you to do?

easy

A. Backup the entire database automatically

B. Listen to real-time insert, update, and delete events on a collection

C. Run queries faster by caching results

D. Create indexes on collections

Solution

1. Step 1: Understand the purpose of change streams

   Change streams provide a way to watch for changes like inserts, updates, and deletes in real time.

2. Step 2: Compare options with the purpose

   Only Listen to real-time insert, update, and delete events on a collection describes listening to these real-time events, while others describe unrelated features.

3. Final Answer:

   Listen to real-time insert, update, and delete events on a collection -> Option B

4. Quick Check:

   Change streams = real-time event listening [OK]

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

Common Mistakes:

• Confusing change streams with backups
• Thinking change streams speed up queries
• Mixing change streams with indexing

2. Which of the following is the correct way to start watching a collection for changes in MongoDB?

easy

A. db.collection.observe()

B. db.collection.listen()

C. db.collection.watch()

D. db.collection.monitor()

Solution

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

   The official method to watch changes is watch() on a collection object.

2. Step 2: Verify options

   Only db.collection.watch() uses the correct method name watch(). Others are invalid method names.

3. Final Answer:

   db.collection.watch() -> Option C

4. Quick Check:

   Correct method = watch() [OK]

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

Common Mistakes:

• Using listen() or observe() which don't exist
• Confusing with monitor() which is not a method
• Forgetting to call watch() on the collection

3. Given this code snippet, what type of change event will be logged when a document is deleted?

const changeStream = db.collection('users').watch();
changeStream.on('change', (change) => {
  console.log(change.operationType);
});
// Then a document is deleted from 'users'

medium

A. "delete"

B. "update"

C. "insert"

D. "replace"

Solution

1. Step 1: Understand operationType values in change streams

   When a document is deleted, the operationType is "delete".

2. Step 2: Match the event with the logged output

   The code logs change.operationType, so deleting a document logs "delete".

3. Final Answer:

   "delete" -> Option A

4. Quick Check:

   Delete event logs "delete" [OK]

Hint: Delete event = operationType "delete" [OK]

Common Mistakes:

• Confusing delete with update or insert
• Expecting 'remove' instead of 'delete'
• Not knowing operationType values

4. You wrote this code to watch changes but it throws an error:

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

What is the error?

medium

A. Using 'const' instead of 'let' for changeStream

B. Incorrect event name 'change'

C. No error, code is correct

D. Missing parentheses after watch() method call

Solution

1. Step 1: Identify how watch() is called

   The code assigns db.collection('orders').watch without parentheses, so it assigns the function itself, not the stream.

2. Step 2: Understand the error cause

   Without calling watch() with parentheses, changeStream is not a stream and has no on method, causing an error.

3. Final Answer:

   Missing parentheses after watch() method call -> Option D

4. Quick Check:

   Call watch() with () to get stream [OK]

Hint: Always call watch() with parentheses () [OK]

Common Mistakes:

• Forgetting parentheses after watch
• Thinking event name 'change' is wrong
• Confusing const vs let for streams

5. You want to listen only for insert events on a collection using change streams. Which of these code snippets correctly filters for inserts only?

const changeStream = db.collection('products').watch([
  { $match: { 'operationType': 'insert' } }
]);
changeStream.on('change', (change) => {
  console.log(change.fullDocument);
});

hard

A. The code correctly filters for insert events only

B. The $match stage should use 'operation_type' instead of 'operationType'

C. You must filter inside the on('change') callback, not in watch()

D. Change streams do not support filtering by operationType

Solution

1. Step 1: Understand change stream pipeline filtering

   You can pass an aggregation pipeline to watch() to filter events, using $match on 'operationType'.

2. Step 2: Verify the filter syntax

   The code uses { $match: { 'operationType': 'insert' } }, which is the correct way to filter for insert events.

3. Final Answer:

   The code correctly filters for insert events only -> Option A

4. Quick Check:

   Filter inserts with $match on operationType [OK]

Hint: Filter inserts using $match on operationType in watch() [OK]

Common Mistakes:

• Using wrong field name like operation_type
• Filtering inside callback instead of pipeline
• Thinking filtering is not supported