Schema design for read-heavy workloads in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a collection optimized for fast reads by embedding related data.

MongoDB

db.users.insertOne({ name: "Alice", orders: [1] })

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

Anull

B[{ item: "book", qty: 2 }, { item: "pen", qty: 5 }]

CObjectId("507f1f77bcf86cd799439011")

D"orders"

Attempts:

3 left

💡 Hint

Common Mistakes

Referencing orders by ID instead of embedding causes slower reads.

Using null or unrelated strings does not store order data.

Embedding the orders array inside the user document allows faster reads by avoiding joins.

2fill in blank

medium

Complete the query to find users with embedded orders containing item 'book'.

MongoDB

db.users.find({ "orders.item": [1] })

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

A"notebook"

B"pen"

C"book"

D"pencil"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong item name returns no results.

Querying on the wrong field does not find embedded documents.

Querying with "orders.item": "book" finds users whose embedded orders include a book.

3fill in blank

hard

Fix the error in the update to add a new embedded order to user 'Alice'.

MongoDB

db.users.updateOne({ name: "Alice" }, { [1]: { orders: { item: "notebook", qty: 3 } } })

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

A$push

B$set

C$addToSet

D$inc

Attempts:

3 left

💡 Hint

Common Mistakes

Using $set replaces the entire orders array instead of adding.

Using $inc is for numbers, not arrays.

$push adds a new element to the embedded orders array, which is needed to add an order.

4fill in blank

hard

Fill both blanks to create an index on the embedded field for faster reads.

MongoDB

db.users.createIndex({ [1]: [2] })

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

Aorders.item

B1

C-1

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Indexing the wrong field does not improve query speed.

Using -1 creates descending index, which is less common here.

Creating an ascending index on 'orders.item' speeds up queries filtering by that embedded field.

5fill in blank

hard

Fill all three blanks to write an aggregation pipeline that unwinds orders and filters by qty > 2.

MongoDB

db.users.aggregate([ { $unwind: "$[1]" }, { $match: { "[2].qty": { [3]: 2 } } } ])

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

Aorders

C$gt

D$lt

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lt filters for less than, which is incorrect here.

Not unwinding the array prevents filtering individual orders.

$unwind deconstructs the orders array, then $match filters orders with qty greater than 2 using $gt.

Practice

1. Which schema design approach is best for read-heavy workloads in MongoDB?

easy

A. Embed related data in the same document to reduce read operations

B. Use references for all data to keep documents small

C. Normalize data like in relational databases

D. Store all data in separate collections regardless of usage

Solution

1. Step 1: Understand read-heavy workload needs

   Read-heavy workloads benefit from fewer queries and faster data retrieval.

2. Step 2: Choose schema design for fast reads

   Embedding related data in one document reduces the need for multiple queries, speeding up reads.

3. Final Answer:

   Embed related data in the same document to reduce read operations -> Option A

4. Quick Check:

   Embedding = faster reads [OK]

Hint: Embed related data for fast reads in read-heavy apps [OK]

Common Mistakes:

• Choosing referencing for all data slows reads
• Normalizing like SQL increases read complexity
• Separating all data causes many queries

2. Which of the following is the correct way to embed an array of comments inside a blog post document in MongoDB?

easy

A. { title: 'Post', comments: [{ user: 'Alice', text: 'Nice!' }, { user: 'Bob', text: 'Great!' }] }

B. { title: 'Post', comments: 'user: Alice, text: Nice!' }

C. { title: 'Post', comments: { user: 'Alice', text: 'Nice!' } }

D. { title: 'Post', comments: ['user: Alice', 'text: Nice!'] }

Solution

1. Step 1: Identify correct array embedding syntax

   Embedding an array of subdocuments requires an array of objects inside the field.

2. Step 2: Check each option's structure

   { title: 'Post', comments: [{ user: 'Alice', text: 'Nice!' }, { user: 'Bob', text: 'Great!' }] } correctly uses an array of objects for comments; others use incorrect types or formats.

3. Final Answer:

   { title: 'Post', comments: [{ user: 'Alice', text: 'Nice!' }, { user: 'Bob', text: 'Great!' }] } -> Option A

4. Quick Check:

   Array of objects = correct embedding [OK]

Hint: Use array of objects to embed multiple related items [OK]

Common Mistakes:

• Using string instead of array for multiple comments
• Embedding a single object instead of an array
• Using array of strings instead of objects

3. Given this MongoDB document for a product with embedded reviews:

{ _id: 1, name: 'Gadget', reviews: [{ user: 'Ann', rating: 5 }, { user: 'Ben', rating: 4 }] }

What will be the result of this query?

db.products.findOne({ _id: 1 }, { name: 1, 'reviews.rating': 1 })

medium

A. { name: 'Gadget', reviews: [{ rating: 5 }, { rating: 4 }] }

B. { _id: 1, name: 'Gadget', reviews: [{ rating: 5 }, { rating: 4 }] }

C. { _id: 1, name: 'Gadget' }

D. { _id: 1, name: 'Gadget', reviews: [{ user: 'Ann', rating: 5 }, { user: 'Ben', rating: 4 }] }

Solution

1. Step 1: Understand projection with dot notation

   Using 'reviews.rating': 1 includes only the rating field inside each review subdocument.

2. Step 2: Analyze returned fields

   The query returns _id, name, and reviews with only rating fields (user excluded).

3. Final Answer:

   { _id: 1, name: 'Gadget', reviews: [{ rating: 5 }, { rating: 4 }] } -> Option B

4. Quick Check:

   Projection with dot notation = filtered subfields [OK]

Hint: Use dot notation in projection to include subfields only [OK]

Common Mistakes:

• Expecting full subdocuments when projecting subfields
• Forgetting _id is included by default
• Assuming projection excludes _id without explicit exclusion

4. You designed a schema embedding user addresses inside user documents, but queries on addresses are slow. What is the likely problem?

medium

A. MongoDB does not support embedding arrays

B. Embedding always makes queries slower

C. You must store addresses in a separate database

D. Addresses should be referenced separately if they are large or updated often

Solution

1. Step 1: Identify embedding drawbacks

   Embedding large or frequently changing data can cause large documents and slow updates or queries.

2. Step 2: Choose referencing for large or mutable data

   Referencing addresses separately keeps user documents small and improves query speed on addresses.

3. Final Answer:

   Addresses should be referenced separately if they are large or updated often -> Option D

4. Quick Check:

   Large mutable data = use referencing [OK]

Hint: Reference large or changing data to keep documents small [OK]

Common Mistakes:

• Assuming embedding always improves speed
• Thinking MongoDB disallows embedding arrays
• Believing separate databases are needed for performance

5. You have a read-heavy app showing user profiles with many posts. Posts change often, but profiles rarely do. How should you design the schema for fast reads?

hard

A. Embed user profile data inside each post document

B. Embed posts inside user profiles despite frequent changes

C. Store user profiles and posts in separate collections, referencing posts from profiles

D. Duplicate posts data inside profiles and update both on changes

Solution

1. Step 1: Analyze data change frequency

   Posts change often; embedding them inside profiles would cause large documents and frequent updates.

2. Step 2: Choose referencing for frequently changing large data

   Storing posts separately and referencing them keeps profiles small and reads fast.

3. Step 3: Avoid data duplication to prevent sync issues

   Duplicating posts in profiles causes update complexity and inconsistency risks.

4. Final Answer:

   Store user profiles and posts in separate collections, referencing posts from profiles -> Option C

5. Quick Check:

   Frequent changes = referencing large data [OK]

Hint: Reference frequently changing data, embed stable data [OK]

Common Mistakes:

• Embedding frequently changing posts inside profiles
• Duplicating data causing sync problems
• Embedding profiles inside posts which is illogical