Anti-patterns to avoid in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this MongoDB query?

Consider a collection users with documents containing name and age. What will this query return?

MongoDB

db.users.find({age: {$gt: 30}}).limit(2)

A Syntax error due to incorrect query

B Returns all users with age greater than 30

C Returns 2 users regardless of age

D Returns the first 2 users with age greater than 30

💡 Hint

Remember that limit() restricts the number of documents returned.

The query filters users with age > 30 and limits the result to 2 documents.

🧠 Conceptual

intermediate

Why is embedding large arrays in documents an anti-pattern?

In MongoDB, what is a major problem with embedding very large arrays inside a single document?

A It improves query speed by avoiding joins

B It can cause the document size to exceed the 16MB limit, leading to errors

C It automatically shards the data across servers

D It compresses the data to save space

💡 Hint

Think about MongoDB's maximum document size.

MongoDB documents have a 16MB size limit. Large arrays can exceed this, causing errors.

📝 Syntax

advanced

Which option causes a syntax error in MongoDB aggregation?

Identify the aggregation pipeline stage that will cause a syntax error.

MongoDB

db.orders.aggregate([
  {$match: {status: 'shipped'}},
  {$group: {_id: '$customerId', total: {$sum: '$amount'}}},
  {$sort: {total: -1}}
])

A {$group: {_id: '$customerId', total: {$sum: 'amount'}}}

B {$group: {_id: '$customerId', total: {$sum: '$amount'}}}

C {$sort: {total: -1}}

D {$match: {status: 'shipped'}}

💡 Hint

Check the syntax of the $sum operator argument.

The $sum operator requires a field path starting with '$'. Missing '$' causes syntax error.

❓ optimization

advanced

Which indexing strategy avoids the anti-pattern of slow queries?

Given a collection with frequent queries filtering by category and sorting by price, which index is best to optimize performance?

A Create a compound index on <code>category</code> and <code>price</code>

B Create a single-field index on <code>price</code>

C Create a single-field index on <code>category</code>

D No index needed; rely on collection scan

💡 Hint

Think about queries that filter and sort on multiple fields.

A compound index on both fields supports filtering and sorting efficiently, avoiding slow queries.

🔧 Debug

expert

Why does this MongoDB update not modify any documents?

Given the collection products and this update command, why are no documents updated?

MongoDB

db.products.updateMany({tags: 'sale'}, {$set: {discount: 10}})

A The <code>$set</code> operator is used incorrectly

B The update command syntax is invalid

C The <code>tags</code> field is an array, so matching a string directly fails

D The <code>updateMany</code> method requires a third argument

💡 Hint

Consider how MongoDB matches array fields in queries.

Matching a string directly to an array field requires using $elemMatch or the string itself if exact match is intended. If tags is an array, matching 'sale' as a string may fail if the array contains objects or different types.