Array of embedded documents queries in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents with an embedded array containing a specific value

Given a collection students where each document has an array field grades with embedded documents like { subject: "Math", score: 85 }, which query returns all students who have a Math score greater than 90?

MongoDB

db.students.find({ grades: { $elemMatch: { subject: "Math", score: { $gt: 90 } } } })

A db.students.find({ grades: { $elemMatch: { subject: "Math", score: { $gt: 90 } } } })

B db.students.find({ grades: { subject: "Math", score: { $gt: 90 } } })

C db.students.find({ "grades.subject": "Math", "grades.score": { $gt: 90 } })

D db.students.find({ grades: { $all: [ { subject: "Math" }, { score: { $gt: 90 } } ] } })

💡 Hint

Use $elemMatch to match multiple conditions inside the same embedded document in the array.

Option A correctly uses $elemMatch to find array elements where both conditions (subject is 'Math' and score > 90) are true in the same embedded document. Option A matches documents where any grade has subject 'Math' and any grade has score > 90, but not necessarily in the same embedded document. Option A is invalid syntax because $elemMatch is missing. Option A uses $all incorrectly for embedded documents.

❓ query_result

intermediate

Count documents with an embedded array matching a condition

In a products collection, each document has an array reviews with embedded documents like { user: "Alice", rating: 5 }. Which query counts how many products have at least one review with rating 5?

MongoDB

db.products.countDocuments({ reviews: { $elemMatch: { rating: 5 } } })

A db.products.countDocuments({ reviews: { $all: [ { rating: 5 } ] } })

B db.products.countDocuments({ reviews: { $elemMatch: { rating: 5 } } })

C db.products.countDocuments({ reviews: { rating: 5 } })

D db.products.countDocuments({ "reviews.rating": 5 })

💡 Hint

Use $elemMatch to ensure the rating 5 is in the same embedded document.

Option B correctly counts documents where at least one embedded review has rating 5 using $elemMatch. Option B matches documents where any review has rating 5 but may count duplicates or not be precise for embedded docs. Option B is invalid syntax. Option B misuses $all for embedded documents.

📝 Syntax

advanced

Identify the syntax error in this embedded document query

Which option contains a syntax error when querying documents with an embedded array items where each item has name and qty fields, to find documents with an item named 'apple' and quantity greater than 10?

MongoDB

db.orders.find({ items: { $elemMatch: { name: 'apple', qty: { $gt: 10 } } } })

A db.orders.find({ items: { $elemMatch: { name: 'apple', qty: { $gt: 10 } } } })

B db.orders.find({ items: { $elemMatch: { name: 'apple', qty: { $gte: 10 } } } })

C db.orders.find({ 'items.name': 'apple', 'items.qty': { $gt: 10 } })

D db.orders.find({ items: { $elemMatch: { name: 'apple', qty: $gt: 10 } } })

💡 Hint

Check how operators like $gt are used inside queries.

Option D is invalid because $gt must be inside an object: { $gt: 10 }, not qty: $gt: 10. Options A, B, and C are syntactically correct queries.

❓ optimization

advanced

Optimize query to find documents with multiple conditions on embedded array

You want to find documents in orders where the products array contains an item with category 'electronics' and price less than 100. Which query is the most efficient and correct?

A db.orders.find({ products: { $all: [ { category: 'electronics' }, { price: { $lt: 100 } } ] } })

B db.orders.find({ 'products.category': 'electronics', 'products.price': { $lt: 100 } })

C db.orders.find({ products: { $elemMatch: { category: 'electronics', price: { $lt: 100 } } } })

D db.orders.find({ products: { category: 'electronics', price: { $lt: 100 } } })

💡 Hint

Use $elemMatch to ensure both conditions apply to the same embedded document.

Option C uses $elemMatch to match both conditions in the same embedded document efficiently. Option C matches documents where any product has category 'electronics' and any product has price < 100, but not necessarily the same product. Option C misuses $all for embedded documents. Option C is invalid syntax.

🧠 Conceptual

expert

Understanding the difference between $elemMatch and dot notation in embedded array queries

Which statement best explains the difference between querying with { 'comments.author': 'John', 'comments.likes': { $gt: 10 } } and { comments: { $elemMatch: { author: 'John', likes: { $gt: 10 } } } } on a collection where comments is an array of embedded documents?

A The first query matches documents where any comment has author 'John' and any comment has likes > 10, possibly in different comments; the second query matches documents where the same comment has both author 'John' and likes > 10.

B Both queries always return the same documents because they check the same conditions on the comments array.

C The first query is faster because it uses dot notation; the second query is slower because $elemMatch is complex.

D The second query matches documents where all comments have author 'John' and likes > 10; the first query matches if any comment matches either condition.

💡 Hint

Think about whether conditions apply to the same embedded document or different ones.

Option A correctly explains that dot notation conditions can match different elements in the array, while $elemMatch ensures all conditions apply to the same embedded document. Option A is incorrect because results can differ. Option A is incorrect about performance assumptions. Option A misrepresents the behavior of $elemMatch and dot notation.