Watch method for real-time updates in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start watching changes on the collection.

MongoDB

const changeStream = collection.[1]();

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

Asubscribe

Blisten

Cobserve

Dwatch

Attempts:

3 left

💡 Hint

Common Mistakes

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

The watch() method starts a change stream to listen for real-time updates on a collection.

2fill in blank

medium

Complete the code to handle change events from the change stream.

MongoDB

changeStream.on('[1]', next => {
  console.log(next);
});

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

Adata

Bchange

Cevent

Dupdate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'update' which is a type of change but not the event name.

The 'change' event is emitted when a change occurs in the watched collection.

3fill in blank

hard

Fix the error in the code to properly close the change stream when done.

MongoDB

await changeStream.[1]();

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

Aclose

Bend

Cstop

Dterminate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'stop' or 'end' which are not valid methods on change streams.

The close() method properly closes the change stream to release resources.

4fill in blank

hard

Fill both blanks to filter change events to only insert operations.

MongoDB

const pipeline = [
  { $match: { 'operationType': [1] } }
];
const changeStream = collection.watch(pipeline, { [2]: false });

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

A'insert'

BfullDocument

Ctrue

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'true' for fullDocument disables filtering, or wrong operationType string.

The pipeline filters for 'insert' operations only. The option fullDocument: false disables returning the full changed document.

5fill in blank

hard

Fill all three blanks to create a change stream that watches updates and prints the updated fields.

MongoDB

const pipeline = [
  { $match: { 'operationType': [1] } }
];
const changeStream = collection.watch(pipeline);
changeStream.on('[2]', change => {
  console.log(change.updateDescription.[3]);
});

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

A'update'

B'change'

CupdatedFields

DremovedFields

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'removedFields' instead of 'updatedFields' when printing changes.

The pipeline filters for 'update' operations. The event listened to is 'change'. The updatedFields property shows which fields were changed.

Practice

1. What is the main purpose of the watch method in MongoDB?

easy

A. To backup the database automatically

B. To create indexes on collections

C. To listen for real-time changes in the database

D. To delete documents in bulk

Solution

1. Step 1: Understand the watch method's role

   The watch method is designed to monitor changes in the database as they happen.

2. Step 2: Compare with other options

   Backup, indexing, and deleting are unrelated to real-time monitoring.

3. Final Answer:

   To listen for real-time changes in the database -> Option C

4. Quick Check:

   watch method = real-time updates [OK]

Hint: Remember: watch means 'listen' for changes [OK]

Common Mistakes:

• Confusing watch with backup or indexing
• Thinking watch deletes data
• Assuming watch modifies data

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

easy

A. db.orders.watch()

B. db.orders.listen()

C. db.orders.observe()

D. db.orders.track()

Solution

1. Step 1: Recall the correct method name

   The method to watch changes is watch(), not listen, observe, or track.

2. Step 2: Apply method to collection

   Calling watch() on db.orders is the correct syntax.

3. Final Answer:

   db.orders.watch() -> Option A

4. Quick Check:

   Correct method is watch() [OK]

Hint: watch() is the exact method name to monitor changes [OK]

Common Mistakes:

• Using listen(), observe(), or track() which don't exist
• Forgetting parentheses after watch
• Calling watch on the database instead of collection

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

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

medium

A. 'insert'

B. 'update'

C. 'delete'

D. No output

Solution

1. Step 1: Understand the event listener

   The code listens for any change event on the users collection and logs the operationType.

2. Step 2: Identify the operation type for insertion

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

3. Final Answer:

   'insert' -> Option A

4. Quick Check:

   Insert triggers operationType 'insert' [OK]

Hint: Insert operationType is 'insert' in change events [OK]

Common Mistakes:

• Expecting 'update' or 'delete' for insert events
• Thinking no output occurs without explicit fetch
• Confusing operationType with full document

4. Identify the error in this MongoDB watch usage:

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

medium

A. The watch method does not accept an object as argument

B. The options should be passed as the second argument, not as the first argument object

C. fullDocument option should be set to 'default', not 'updateLookup'

D. The on method is not valid for change streams

Solution

1. Step 1: Check watch method parameters

   The watch method accepts an optional array pipeline as the first argument, not an object with options.

2. Step 2: Understand where options go

   Options like fullDocument should be passed as the second argument, not inside the pipeline array.

3. Final Answer:

   The options should be passed as the second argument, not as the first argument object -> Option B

4. Quick Check:

   Pipeline is array, options are separate [OK]

Hint: Pass pipeline as array, options as separate object [OK]

Common Mistakes:

• Passing options as first argument instead of second
• Using invalid values for fullDocument
• Misusing event handlers on change streams

5. You want to watch only update operations on the messages collection and get the full updated document in real-time. Which code snippet correctly achieves this?

hard

A. db.messages.watch([{ $match: { operationType: 'update' } }])

B. db.messages.watch({ operationType: 'update' }, { fullDocument: 'default' })

C. db.messages.watch([{ $match: { operationType: 'update' } }], { fullDocument: 'default' })

D. db.messages.watch([{ $match: { operationType: 'update' } }], { fullDocument: 'updateLookup' })

Solution

1. Step 1: Filter for update operations

   Use a pipeline with $match to watch only update operation types.

2. Step 2: Get full updated document

   Set fullDocument option to 'updateLookup' to receive the full document on update.

3. Step 3: Verify correct syntax

   Pass the pipeline array as first argument and options object as second argument to watch.

4. Final Answer:

   db.messages.watch([{ $match: { operationType: 'update' } }], { fullDocument: 'updateLookup' }) -> Option D

5. Quick Check:

   Pipeline filter + fullDocument 'updateLookup' = db.messages.watch([{ $match: { operationType: 'update' } }], { fullDocument: 'updateLookup' }) [OK]

Hint: Use $match pipeline and fullDocument: 'updateLookup' option [OK]

Common Mistakes:

• Passing options as first argument instead of second
• Using fullDocument: 'default' which doesn't guarantee full doc on update
• Not filtering operationType in pipeline