Array expressions ($size, $arrayElemAt, $filter) in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of $size on an empty array?

Consider the following MongoDB aggregation pipeline stage:

{ $project: { count: { $size: "$items" } } }

If the document is { items: [] }, what will be the value of count in the output?

MongoDB

{ items: [] }

A null

B 0

C 1

D undefined

💡 Hint

Think about how many elements are inside the array.

The $size operator returns the number of elements in the array. An empty array has zero elements, so the result is 0.

❓ query_result

intermediate

Using $arrayElemAt to get the second element

Given the document:

{ fruits: ["apple", "banana", "cherry"] }

What is the output of this aggregation projection?

{ $project: { secondFruit: { $arrayElemAt: ["$fruits", 1] } } }

MongoDB

{ fruits: ["apple", "banana", "cherry"] }

A "cherry"

B "apple"

C "banana"

D null

💡 Hint

Remember that array indexes start at 0.

The $arrayElemAt operator returns the element at the specified index. Index 1 is the second element, which is "banana".

❓ query_result

advanced

Filtering array elements with $filter

Given the document:

{ scores: [85, 42, 90, 70, 55] }

What is the output of this aggregation projection?

{ $project: { highScores: { $filter: { input: "$scores", as: "score", cond: { $gte: ["$$score", 70] } } } } }

MongoDB

{ scores: [85, 42, 90, 70, 55] }

A [42, 55]

B []

C [85, 42, 90, 70, 55]

D [85, 90, 70]

💡 Hint

The condition keeps scores greater than or equal to 70.

The $filter operator returns only elements where the condition is true. Scores 85, 90, and 70 meet the condition.

📝 Syntax

advanced

Identify the syntax error in $filter usage

Which of the following $filter expressions will cause a syntax error in MongoDB aggregation?

A { $filter: { input: "$values", cond: { $gt: ["$$val", 10] } } }

B { $filter: { input: "$values", as: "val", cond: { $lt: ["$$val", 5] } } }

C { $filter: { input: "$values", as: "val", cond: { $eq: ["$$val", 3] } } }

D { $filter: { input: "$values", as: "val", cond: { $gt: ["$$val", 10] } } }

💡 Hint

Check if all required fields are present.

Option A is missing the as field, which defines the variable name for elements in the array. This causes a syntax error.

🧠 Conceptual

expert

Combining $size, $filter, and $arrayElemAt for complex queries

You have documents with a field tags which is an array of strings. You want to find the first tag that starts with the letter 'a' and also know how many tags start with 'a'. Which aggregation expression correctly produces both the first matching tag and the count of matching tags?

A { firstTag: { $arrayElemAt: [ { $filter: { input: "$tags", as: "tag", cond: { $regexMatch: { input: "$$tag", regex: /^a/ } } } }, 0 ] }, count: { $size: { $filter: { input: "$tags", as: "tag", cond: { $regexMatch: { input: "$$tag", regex: /^a/ } } } } } }

B { firstTag: { $arrayElemAt: [ "$tags", 0 ] }, count: { $size: "$tags" } }

C { firstTag: { $filter: { input: "$tags", as: "tag", cond: { $regexMatch: { input: "$$tag", regex: /^a/ } } } }, count: { $size: "$tags" } }

D { firstTag: { $arrayElemAt: [ { $filter: { input: "$tags", cond: { $regexMatch: { input: "$$tag", regex: /^a/ } } } }, 0 ] }, count: { $size: { $filter: { input: "$tags", as: "tag", cond: { $regexMatch: { input: "$$tag", regex: /^a/ } } } } } }

💡 Hint

Remember to define the variable name with 'as' in $filter and use it consistently.

Option A correctly uses as: "tag" to name the variable and applies the regex condition in both $filter calls. It then uses $arrayElemAt to get the first matching tag and $size to count all matching tags.

Option A is missing as in the first $filter, causing an error.

Options B and C do not correctly filter tags starting with 'a'.