Change stream events (insert, update, delete) in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Identify the change stream event type for a document insertion

Given a MongoDB change stream watching a collection, what is the value of the operationType field in the event document when a new document is inserted?

A "update"

B "delete"

C "insert"

D "replace"

💡 Hint

Think about what the event type would be when a new document is added.

When a new document is added to a collection, the change stream event's operationType is "insert".

❓ query_result

intermediate

Determine the change stream event for a document deletion

What is the operationType value in a MongoDB change stream event when a document is deleted from a collection?

A "delete"

B "drop"

C "insert"

D "update"

💡 Hint

Consider the event type that signals removal of a document.

The operationType for a deletion event in a change stream is "delete".

📝 Syntax

advanced

Which change stream event document contains the updated fields?

In a MongoDB change stream event for an update operation, which field contains the changed fields and their new values?

A "documentKey"

B "updateDescription.updatedFields"

C "fullDocument"

D "ns"

💡 Hint

Look for the field that describes what changed inside the document.

The updateDescription.updatedFields field contains the fields that were changed and their new values in an update event.

🧠 Conceptual

advanced

Understanding the difference between 'replace' and 'update' events

Which statement correctly describes the difference between a 'replace' and an 'update' operationType in MongoDB change streams?

A 'replace' means the entire document was replaced; 'update' means only some fields changed

B 'replace' means the document was deleted; 'update' means the document was inserted

C 'replace' and 'update' are synonyms and mean the same thing

D 'replace' means the collection was dropped; 'update' means the document was replaced

💡 Hint

Think about whether the whole document or just parts of it changed.

A 'replace' event means the entire document was replaced with a new one, while an 'update' event means only some fields were changed.

🔧 Debug

expert

Identify the error in this change stream pipeline

You want to watch only delete events on a collection using a change stream with this pipeline:

[ { $match: { operationType: "delete" } } ]

But your code throws an error. What is the most likely cause?

MongoDB

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

A The 'operationType' field should be matched as a number, not a string

B The pipeline must use '$match' with 'fullDocument' instead of 'operationType'

C The 'operationType' field must be inside 'updateDescription' in the match stage

D The pipeline is correct; the error is likely from missing permissions or connection issues

💡 Hint

Check if the pipeline syntax matches MongoDB documentation for filtering by operationType.

Filtering by 'operationType' in the $match stage is valid syntax. Errors usually come from permissions or connection problems, not the pipeline itself.

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