$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.

Practice

1. What does the $size operator do in MongoDB?

easy

A. Counts the number of elements in an array

B. Calculates the sum of numbers in an array

C. Finds the largest number in an array

D. Sorts the elements of an array

Solution

1. Step 1: Understand the purpose of $size

   The $size operator is used to count how many elements are inside an array field in a MongoDB document.

2. Step 2: Compare with other options

   Other options describe different operations like sum, max, or sort, which are not what $size does.

3. Final Answer:

   Counts the number of elements in an array -> Option A

4. Quick Check:

   $size = count array elements [OK]

Hint: Remember: $size counts array items, not values [OK]

Common Mistakes:

• Confusing $size with sum or max functions
• Thinking $size sorts arrays
• Using $size on non-array fields

2. Which of the following is the correct syntax to use $size in a MongoDB aggregation pipeline to add a field itemCount that counts elements in the items array?

easy

A. { $addFields: { itemCount: { $length: "$items" } } }

B. { $match: { itemCount: { $size: "$items" } } }

C. { $project: { itemCount: { $size: "items" } } }

D. { $addFields: { itemCount: { $size: "$items" } } }

Solution

1. Step 1: Identify correct operator usage in aggregation

   The $size operator is used inside an expression to count array elements. It must be inside a stage like $addFields or $project with the array field referenced as "$items".

2. Step 2: Check syntax correctness

   { $addFields: { itemCount: { $length: "$items" } } } uses a non-existent $length. { $project: { itemCount: { $size: "items" } } } misses the $ before items. { $match: { itemCount: { $size: "$items" } } } misuses $match with $size.

3. Final Answer:

   { $addFields: { itemCount: { $size: "$items" } } } -> Option D

4. Quick Check:

   Use $size inside $addFields with "$arrayField" [OK]

Hint: Use "$arrayField" inside $size in $addFields or $project [OK]

Common Mistakes:

• Using $length instead of $size
• Forgetting the $ before array field name
• Using $size inside $match incorrectly

3. Given the collection documents:

{ "name": "Alice", "tags": ["red", "blue"] }

{ "name": "Bob", "tags": ["green"] }

{ "name": "Carol", "tags": [] }

What will be the result of this aggregation pipeline?

[{ $project: { name: 1, tagCount: { $size: "$tags" } } }]

medium

A. [{ "name": "Alice", "tagCount": 2 }, { "name": "Bob", "tagCount": 1 }, { "name": "Carol", "tagCount": 0 }]

B. [{ "name": "Alice", "tagCount": 3 }, { "name": "Bob", "tagCount": 1 }, { "name": "Carol", "tagCount": 1 }]

C. [{ "name": "Alice", "tagCount": 2 }, { "name": "Bob", "tagCount": 0 }, { "name": "Carol", "tagCount": 0 }]

D. SyntaxError

Solution

1. Step 1: Understand $size counts array elements

   For each document, $size counts how many items are in the tags array: Alice has 2, Bob has 1, Carol has 0.

2. Step 2: Apply $project to include name and tagCount

   The pipeline projects the name and adds tagCount with the counted size.

3. Final Answer:

   [{ "name": "Alice", "tagCount": 2 }, { "name": "Bob", "tagCount": 1 }, { "name": "Carol", "tagCount": 0 }] -> Option A

4. Quick Check:

   Count array lengths with $size = correct counts [OK]

Hint: Count array length per document with $size in $project [OK]

Common Mistakes:

• Assuming empty arrays count as 1
• Mixing up counts for different documents
• Expecting syntax error for correct query

4. You wrote this aggregation stage to filter documents with exactly 3 tags:

{ $match: { tags: { $size: 3 } } }

But it returns an error. What is the problem?

medium

A. The array field name is missing the $ sign

B. The $match stage requires $expr to use $size

C. The number 3 should be in quotes as "3"

D. $size cannot be used inside $match like this

Solution

1. Step 1: Understand $size usage in $match

   Directly using $size inside $match like this is invalid because $size is an aggregation expression, not a query operator.

2. Step 2: Use $expr to evaluate aggregation expressions in $match

   To filter by array length, you must use $expr with $size, like: { $match: { $expr: { $eq: [ { $size: "$tags" }, 3 ] } } }.

3. Final Answer:

   The $match stage requires $expr to use $size -> Option B

4. Quick Check:

   Use $expr for aggregation expressions in $match [OK]

Hint: Use $expr to apply $size inside $match [OK]

Common Mistakes:

• Trying to use $size directly in $match
• Forgetting $expr wrapper
• Using quotes around numbers incorrectly

5. You want to find documents where the comments array has more than 2 elements. Which aggregation pipeline stage correctly filters these documents?

hard

A. { $match: { $size: { $gt: [ "$comments", 2 ] } } }

B. { $match: { comments: { $size: { $gt: 2 } } } }

C. { $match: { $expr: { $gt: [ { $size: "$comments" }, 2 ] } } }

D. { $match: { $expr: { $size: { $gt: [ "$comments", 2 ] } } } }

Solution

1. Step 1: Use $expr to evaluate expressions in $match

   To compare array length, use $expr to allow aggregation expressions inside $match.

2. Step 2: Use $gt with $size to check array length greater than 2

   The correct syntax is { $gt: [ { $size: "$comments" }, 2 ] } inside $expr.

3. Step 3: Eliminate incorrect options

   The incorrect options either lack $expr, misuse $size placement, or have wrong syntax for $gt.

4. Final Answer:

   { $match: { $expr: { $gt: [ { $size: "$comments" }, 2 ] } } } -> Option C

5. Quick Check:

   Filter by array length with $expr and $gt [OK]

Hint: Use $expr with $gt and $size to filter by array length [OK]

Common Mistakes:

• Using $size as a query operator inside $match
• Wrong order or structure of $gt and $size
• Missing $expr wrapper