Change streams on databases in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to watch changes on the entire database.

MongoDB

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

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

Adatabase

Bwatch

CwatchStream

Dcollection

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'collection' instead of 'database' to watch changes on the whole database.

Trying to call 'watch' directly on 'db' without specifying 'database'.

To watch changes on the whole database, use db.database.watch().

2fill in blank

medium

Complete the code to listen for change events and print them.

MongoDB

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

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

Aevent

Bnext

Cdata

Dchange

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' or 'data' as event names instead of 'change'.

Not using quotes around the event name.

The event name to listen for changes is 'change'.

3fill in blank

hard

Fix the error in the code to start watching changes on a database named 'shopDB'.

MongoDB

const changeStream = client.db('[1]').watch();

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

Ashop

BShopDB

CshopDB

Dshop_db

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'shop' or 'ShopDB' which do not match the exact database name.

Using underscores instead of camel case.

The database name must match exactly, including case. Here it is 'shopDB'.

4fill in blank

hard

Fill both blanks to filter change events to only 'insert' operations on the database.

MongoDB

const pipeline = [ { $match: { 'operationType': [1] } } ];
const changeStream = db.database.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' instead of 'false' for fullDocument option.

Not quoting the operationType string.

The filter uses 'insert' for operationType, and fullDocument: false disables full document retrieval.

5fill in blank

hard

Fill all three blanks to close the change stream properly after use.

MongoDB

async function watchAndClose() {
  const changeStream = db.database.watch();
  changeStream.on('change', data => console.log(data));
  await changeStream.[1]();
  console.log('[2] closed');
  return [3];
}

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

Aclose

BChange stream

Ctrue

Dstop

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'stop' instead of 'close' to end the stream.

Returning a string instead of a boolean.

Use close() to stop the stream, print a message, and return true to indicate success.

Practice

1. What is the main purpose of MongoDB change streams on databases?

easy

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

B. To backup the database automatically

C. To create indexes on collections

D. To run queries faster

Solution

1. Step 1: Understand what change streams do

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

2. Step 2: Compare options with this purpose

   Only To listen for real-time changes in the database describes listening to real-time changes, which matches the main purpose of change streams.

3. Final Answer:

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

4. Quick Check:

   Change streams = real-time listening [OK]

Hint: Change streams = real-time database updates listener [OK]

Common Mistakes:

• Confusing change streams with backups
• Thinking change streams create indexes
• Assuming change streams speed up queries

2. Which of the following is the correct syntax to start watching changes on a MongoDB database named mydb?

easy

A. watch.db('mydb')

B. db.watch('mydb')

C. db.mydb.watch()

D. db.getSiblingDB('mydb').watch()

Solution

1. Step 1: Recall how to access a database in MongoDB shell

   To access a database other than the current one, use db.getSiblingDB('mydb').

2. Step 2: Apply the watch() method on the database object

   The watch() method is called on the database object to start change streams.

3. Final Answer:

   db.getSiblingDB('mydb').watch() -> Option D

4. Quick Check:

   Use getSiblingDB to access db, then watch() [OK]

Hint: Use db.getSiblingDB('name').watch() to watch a database [OK]

Common Mistakes:

• Trying to call watch() directly on db with database name
• Using incorrect method like db.watch('name')
• Confusing method order or syntax

3. Given the following code snippet, what will be the output if a document is inserted into any collection in the testdb database?

const changeStream = db.getSiblingDB('testdb').watch();
changeStream.forEach((change) => {
  printjson(change.operationType);
});

medium

A. "update"

B. "delete"

C. "insert"

D. No output

Solution

1. Step 1: Understand what triggers the change event

   The change stream listens to all changes in the database, including inserts, updates, and deletes.

2. Step 2: Identify the operationType for an insert

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

3. Final Answer:

   "insert" -> Option C

4. Quick Check:

   Insert triggers operationType = "insert" [OK]

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

Common Mistakes:

• Expecting 'update' or 'delete' for insert events
• Thinking no output occurs without filters
• Confusing event names with method names

4. Identify the error in this code snippet that tries to watch changes on the sales database:

const changeStream = db.sales.watch();
changeStream.forEach((change) => {
  printjson(change);
});

medium

A. The watch() method cannot be called on a database object

B. The database should be accessed using getSiblingDB('sales') before calling watch()

C. The on() method is not valid for change streams

D. Missing parentheses after watch

Solution

1. Step 1: Check how the database is accessed

   Using db.sales accesses a collection named 'sales' in the current database, not the database itself.

2. Step 2: Correct way to access a different database

   To watch changes on the 'sales' database, use db.getSiblingDB('sales').watch().

3. Final Answer:

   The database should be accessed using getSiblingDB('sales') before calling watch() -> Option B

4. Quick Check:

   Use getSiblingDB to access database, not db.sales [OK]

Hint: Use getSiblingDB('name') to access database, not db.name [OK]

Common Mistakes:

• Assuming db.sales is a database instead of a collection
• Thinking watch() can't be called on database objects
• Confusing event handling methods

5. You want to watch only update operations on the orders database using change streams. Which code snippet correctly applies a filter to achieve this?

hard

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

B. db.orders.watch({ operationType: 'update' })

C. db.getSiblingDB('orders').watch({ filter: { operationType: 'update' } })

D. db.getSiblingDB('orders').watch([{ $filter: { operationType: 'update' } }])

Solution

1. Step 1: Recall how to filter change streams

   Change streams accept an aggregation pipeline array to filter events, using stages like $match.

2. Step 2: Apply correct syntax for filtering updates

   The correct filter is [{ $match: { operationType: 'update' } }] passed as an array to watch().

3. Final Answer:

   db.getSiblingDB('orders').watch([{ $match: { operationType: 'update' } }]) -> Option A

4. Quick Check:

   Use $match in array to filter change streams [OK]

Hint: Use watch([{ $match: { operationType: 'update' } }]) to filter updates [OK]

Common Mistakes:

• Passing filter as object instead of pipeline array
• Using invalid keys like 'filter' or '$filter'
• Calling watch() on collection instead of database