$size operator for array length in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents with array length exactly 3

Given a collection students where each document has a field courses which is an array of course names, which query returns all students enrolled in exactly 3 courses?

MongoDB

db.students.find({ courses: { $size: 3 } })

A db.students.find({ $size: { courses: 3 } })

B db.students.find({ courses: { $size: 3 } })

C db.students.find({ courses: { $length: 3 } })

D db.students.find({ courses: { $count: 3 } })

💡 Hint

Use the $size operator inside the query to match array length.

The $size operator matches arrays with the exact specified length. Only option B uses $size correctly.

🧠 Conceptual

intermediate

Understanding $size operator behavior

What does the $size operator do in a MongoDB query?

A Filters documents where the array contains at least the given number of elements.

B Counts the total number of documents in the collection.

C Returns the length of a string field in the document.

D Matches documents where the specified array field has exactly the given number of elements.

💡 Hint

Think about how $size relates to arrays.

$size matches documents where the array field length equals the specified number exactly.

📝 Syntax

advanced

Identify the correct aggregation pipeline stage using $size

Which aggregation pipeline stage correctly adds a new field numTags that contains the length of the tags array in each document?

MongoDB

db.posts.aggregate([ { $addFields: { numTags: ??? } } ])

A { $size: "$tags" }

B { $length: "$tags" }

C { $count: "$tags" }

D { $size: tags }

💡 Hint

Use $size with the array field as a string path.

In aggregation, $size takes a field path as a string to compute array length. Only option A is correct syntax.

🔧 Debug

advanced

Why does this query fail?

Consider the query: db.orders.find({ items: { $size: "3" } }). Why does this query not return any documents?

A Because the collection 'orders' does not exist.

B Because $size cannot be used inside find queries.

C Because $size expects a number, not a string, so "3" is invalid.

D Because the field name 'items' is misspelled.

💡 Hint

Check the data type used with $size.

$size requires a numeric value to specify array length. Using a string causes no matches.

❓ optimization

expert

Optimizing queries with $size on large collections

You have a large collection with millions of documents. You want to find documents where the comments array has exactly 5 elements. Which approach is most efficient?

A Create an index on the <code>comments</code> field and use <code>{ comments: { $size: 5 } }</code> in the query.

B Use <code>{ comments: { $size: 5 } }</code> without any index.

C Use <code>{ comments: { $exists: true } }</code> and filter array length in application code.

D Use <code>{ $expr: { $eq: [ { $size: "$comments" }, 5 ] } }</code> without any index.

💡 Hint

Indexes help speed up queries on large data.

MongoDB supports multikey indexes on array fields. Indexing comments allows efficient $size queries.