$in for matching a set in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents where a field matches any value in a set

Given a collection products with documents containing a category field, which query returns all products whose category is either 'electronics', 'books', or 'clothing'?

MongoDB

db.products.find({ category: { $in: ['electronics', 'books', 'clothing'] } })

A Returns products with category exactly matching the array ['electronics', 'books', 'clothing']

B Returns all products with category 'electronics', 'books', or 'clothing'

C Returns products with category not in 'electronics', 'books', or 'clothing'

D Returns all products regardless of category

💡 Hint

Think about how $in matches any value inside the array.

The $in operator matches documents where the field's value is any one of the values in the specified array. So it returns products whose category is 'electronics', 'books', or 'clothing'.

❓ query_result

intermediate

Using $in with numeric values

Consider a collection orders where each document has a numeric status_code. Which query returns orders with status codes 1, 3, or 5?

MongoDB

db.orders.find({ status_code: { $in: [1, 3, 5] } })

A Returns all orders regardless of status_code

B Returns orders with status_code exactly equal to the array [1, 3, 5]

C Returns orders with status_code 1, 3, or 5

D Returns orders with status_code not equal to 1, 3, or 5

💡 Hint

Remember $in checks if the field value is in the array.

The $in operator matches documents where the status_code is any of the values 1, 3, or 5.

📝 Syntax

advanced

Identify the syntax error in $in usage

Which of the following MongoDB queries has a syntax error when using $in to match the tags field?

A db.collection.find({ tags: { $in: 'red', 'blue', 'green' } })

B db.collection.find({ tags: { $in: ['red', 'blue', 'green'] } })

C )} } ]'neerg' ,'eulb' ,'der'[ :ni$ { :sgat {(dnif.noitcelloc.bd

D db.collection.find({ tags: { $in: ['red'] } })

💡 Hint

Check how the array is passed to $in.

The $in operator requires an array as its value. Option A incorrectly passes multiple string arguments instead of an array, causing a syntax error.

❓ optimization

advanced

Optimizing queries with $in on indexed fields

You have a large collection users with an index on the role field. Which query will best use the index to find users with roles 'admin', 'editor', or 'viewer'?

A db.users.find({ $or: [{ role: 'admin' }, { role: 'editor' }, { role: 'viewer' }] })

B db.users.find({ role: { $all: ['admin', 'editor', 'viewer'] } })

C db.users.find({ role: { $nin: ['admin', 'editor', 'viewer'] } })

D db.users.find({ role: { $in: ['admin', 'editor', 'viewer'] } })

💡 Hint

Think about how $in uses indexes compared to $or.

The $in operator efficiently uses indexes on the field to match any of the specified values. While $or can also use indexes, $in is more concise and optimized for this use case.

🧠 Conceptual

expert

Understanding $in behavior with arrays in documents

In a collection posts, each document has a field tags which is an array of strings. What does the query db.posts.find({ tags: { $in: ['mongodb', 'database'] } }) return?

A Documents where the tags array contains at least one of 'mongodb' or 'database'

B Documents where the tags array exactly matches ['mongodb', 'database']

C Documents where the tags array contains both 'mongodb' and 'database'

D Documents where the tags array contains neither 'mongodb' nor 'database'

💡 Hint

Remember how $in works with array fields in documents.

The $in operator matches if any element of the tags array is in the specified list. It does not require all elements to match or exact array equality.