Change stream pipelines for filtering in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a change stream that listens for all changes.

MongoDB

const changeStream = collection.watch([1]);

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

Anull

B{}

C[]

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using an object {} instead of an array []

Passing null or true which are invalid pipeline arguments

The watch method takes an array as the pipeline argument. An empty array [] means no filtering, so all changes are listened to.

2fill in blank

medium

Complete the code to filter change events to only include inserts.

MongoDB

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

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

A'delete'

B'update'

C'replace'

D'insert'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'update' or 'delete' which filter other change types

Forgetting to put quotes around the string

The operationType field indicates the type of change. To filter for inserts, use 'insert'.

3fill in blank

hard

Fix the error in the pipeline to filter changes where the field 'status' is 'active'.

MongoDB

const pipeline = [ { $match: { 'fullDocument.status': [1] } } ];

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

A'active'

Bactive

C"status"

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using unquoted strings causing syntax errors

Using boolean true instead of the string

String values in MongoDB queries must be quoted. So 'active' is correct.

4fill in blank

hard

Fill both blanks to filter change events for updates where the 'age' field is greater than 30.

MongoDB

const pipeline = [ { $match: { operationType: [1], 'fullDocument.age': { [2]: 30 } } } ];

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

A'update'

B'insert'

C$gt

D$lt

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'insert' instead of 'update' for operationType

Using $lt instead of $gt for greater than comparison

To filter for update events, use 'update'. To check if 'age' is greater than 30, use the MongoDB operator $gt.

5fill in blank

hard

Fill all three blanks to create a pipeline that filters for delete operations where the document's 'score' is less than 50.

MongoDB

const pipeline = [ { $match: { operationType: [1], 'fullDocument.score': { [2]: 50 }, 'ns.db': [3] } } ];

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

A'insert'

B$lt

C'myDatabase'

D'delete'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'insert' instead of 'delete' for operationType

Using $gt instead of $lt for less than comparison

Not quoting the database name

To filter delete operations, use 'delete'. To check if 'score' is less than 50, use $lt. The database name is a string, so use 'myDatabase'.

Practice

1. What is the main purpose of using a $match stage in a MongoDB change stream pipeline?

easy

A. To insert new documents into the collection

B. To create indexes on the collection

C. To delete documents from the collection

D. To filter change events and receive only relevant updates

Solution

1. Step 1: Understand change streams

   Change streams allow applications to listen to changes in MongoDB collections or databases.

2. Step 2: Role of $match in pipelines

   The $match stage filters these change events so only specific changes are passed through.

3. Final Answer:

   To filter change events and receive only relevant updates -> Option D

4. Quick Check:

   $match filters events [OK]

Hint: Remember: $match filters events to reduce noise [OK]

Common Mistakes:

• Thinking $match modifies data instead of filtering
• Confusing $match with data insertion or deletion
• Assuming $match creates indexes

2. Which of the following is the correct syntax to filter change stream events for only insert operations using $match?

easy

A. { $match: { operationType: 'insert' } }

B. { $match: { operationType: 'replace' } }

C. { $match: { operationType: 'delete' } }

D. { $match: { operationType: 'update' } }

Solution

1. Step 1: Identify operationType for inserts

   MongoDB change streams use operationType to specify the type of change, and 'insert' is the value for insert operations.

2. Step 2: Match only insert events

   The correct filter uses { $match: { operationType: 'insert' } } to watch only insert changes.

3. Final Answer:

   { $match: { operationType: 'insert' } } -> Option A

4. Quick Check:

   Filter inserts with operationType 'insert' [OK]

Hint: Match operationType: 'insert' to filter inserts [OK]

Common Mistakes:

• Using wrong operationType values like 'update' or 'delete'
• Missing quotes around 'insert'
• Using $match incorrectly outside pipeline

3. Given the following change stream pipeline, what events will be received?

[ { $match: { 'fullDocument.status': 'active' } } ]

medium

A. All change events regardless of document status

B. Only delete events

C. Only change events where the updated document has status 'active'

D. Only insert events with status 'inactive'

Solution

1. Step 1: Understand the filter condition

   The pipeline filters events where the fullDocument.status field equals 'active'.

2. Step 2: Effect on change events

   This means only events where the changed document's status is 'active' will pass through.

3. Final Answer:

   Only change events where the updated document has status 'active' -> Option C

4. Quick Check:

   Filter by fullDocument.status = 'active' [OK]

Hint: Filter on fullDocument fields to get specific document changes [OK]

Common Mistakes:

• Assuming all events pass through without filtering
• Confusing delete events which don't have fullDocument
• Thinking it filters by operationType

4. You wrote this change stream pipeline to watch only delete events:

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

But you still receive update events. What is the likely mistake?

medium

A. The field name should be operation_type instead of operationType

B. The pipeline is not applied correctly or the driver ignores the filter

C. The $match stage is missing in the pipeline

D. Delete events do not have an operationType field

Solution

1. Step 1: Verify field name and filter correctness

   The field operationType is correct and the filter matches delete events properly.

2. Step 2: Consider pipeline application

   If update events still appear, the pipeline might not be applied correctly or the driver ignores the filter.

3. Final Answer:

   The pipeline is not applied correctly or the driver ignores the filter -> Option B

4. Quick Check:

   Incorrect pipeline application causes wrong events [OK]

Hint: Check if pipeline is properly passed to watch() method [OK]

Common Mistakes:

• Using wrong field name like operation_type
• Assuming delete events lack operationType
• Not passing pipeline to watch() function

5. You want to watch changes only for documents where category is 'books' and the operation is either 'insert' or 'update'. Which pipeline correctly filters these events?

hard

A. [ { $match: { $and: [ { 'fullDocument.category': 'books' }, { operationType: { $in: ['insert', 'update'] } } ] } } ]

B. [ { $match: { 'fullDocument.category': 'books', operationType: 'insert' } }, { $match: { operationType: 'update' } } ]

C. [ { $match: { operationType: 'insert' } }, { $match: { 'fullDocument.category': 'books' } } ]

D. [ { $match: { $or: [ { 'fullDocument.category': 'books' }, { operationType: 'insert' }, { operationType: 'update' } ] } } ]

Solution

1. Step 1: Combine conditions with $and

   We want documents where category is 'books' AND operationType is 'insert' or 'update'.

2. Step 2: Use $in for multiple operationTypes

   The correct way is to use $in inside $and to filter operationType for both 'insert' and 'update'.

3. Final Answer:

   [ { $match: { $and: [ { 'fullDocument.category': 'books' }, { operationType: { $in: ['insert', 'update'] } } ] } } ] -> Option A

4. Quick Check:

   Use $and with $in to combine filters [OK]

Hint: Use $and with $in to filter multiple conditions together [OK]

Common Mistakes:

• Using multiple $match stages that override each other
• Using $or instead of $and for combined conditions
• Not using $in for multiple operationTypes