$elemMatch for complex array queries in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find documents where the 'scores' array has an element greater than 80.

MongoDB

db.students.find({ scores: { [1]: { score: { $gt: 80 } } } })

Drag options to blanks, or click blank then click option'

A$and

B$gt

C$elemMatch

D$in

Attempts:

3 left

💡 Hint

Common Mistakes

Using $gt directly on the array field instead of inside $elemMatch.

Using $in which checks for exact values, not conditions.

The $elemMatch operator matches documents that contain an array field with at least one element that matches all the specified query criteria.

2fill in blank

medium

Complete the code to find documents where the 'grades' array has an element with 'score' greater than 85 and 'type' equal to 'exam'.

MongoDB

db.records.find({ grades: { $elemMatch: { score: { [1]: 85 }, type: 'exam' } } })

Drag options to blanks, or click blank then click option'

A$gt

B$lt

C$eq

D$ne

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lt which means less than.

Using $eq which means equal to.

The $gt operator means 'greater than'. Here it checks if the 'score' field in any element is greater than 85.

3fill in blank

hard

Fix the error in the query to find documents where 'items' array has an element with 'price' less than 50 and 'qty' greater than 10.

MongoDB

db.orders.find({ items: { [1]: { price: { $lt: 50 }, qty: { $gt: 10 } } } })

Drag options to blanks, or click blank then click option'

A$and

B$or

C$all

D$elemMatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using $and or $or directly on the array field instead of $elemMatch.

Using $all which matches all elements but not conditions inside elements.

The $elemMatch operator is needed to match documents where an array element satisfies multiple conditions together.

4fill in blank

hard

Fill both blanks to find documents where 'reviews' array has an element with 'rating' at least 4 and 'verified' is true.

MongoDB

db.products.find({ reviews: { [1]: { rating: { [2]: 4 }, verified: true } } })

Drag options to blanks, or click blank then click option'

A$elemMatch

B$gte

C$lt

D$ne

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lt instead of $gte for rating condition.

Using $ne which means not equal.

$elemMatch matches array elements with multiple conditions. $gte means 'greater than or equal to', used here for rating at least 4.

5fill in blank

hard

Fill all three blanks to find documents where 'logs' array has an element with 'status' equal to 'error', 'code' greater than 500, and 'retry' false.

MongoDB

db.system.find({ logs: { [1]: { status: [2], code: { [3]: 500 }, retry: false } } })

Drag options to blanks, or click blank then click option'

A$elemMatch

B'error'

C$gt

D'success'

Attempts:

3 left

💡 Hint

Common Mistakes

Using $and instead of $elemMatch.

Using 'success' instead of 'error' for status.

Using $lt instead of $gt for code.

$elemMatch matches array elements with multiple conditions. The status must be the string 'error'. The code must be greater than 500 using $gt.

Practice

1. What does the $elemMatch operator do in MongoDB queries?

easy

A. Matches documents where any array element matches any one condition.

B. Updates all elements in an array regardless of conditions.

C. Finds array elements that match all specified conditions together.

D. Deletes array elements that do not match the condition.

Solution

1. Step 1: Understand array queries

   MongoDB arrays can contain multiple elements, and queries may need to check multiple conditions on the same element.

2. Step 2: Role of $elemMatch

   $elemMatch ensures all conditions apply to the same array element, not spread across different elements.

3. Final Answer:

   Finds array elements that match all specified conditions together. -> Option C

4. Quick Check:

   $elemMatch = all conditions on one element [OK]

Hint: Use $elemMatch to match multiple conditions on one array item [OK]

Common Mistakes:

• Thinking $elemMatch matches conditions across different elements
• Confusing $elemMatch with $in or $all
• Assuming $elemMatch updates or deletes elements

2. Which of the following is the correct syntax to find documents where an array field scores has an element with score greater than 80 and type equal to 'exam' using $elemMatch?

easy

A. { scores: { $elemMatch: { $gt: 80, type: 'exam' } } }

B. { scores: { $elemMatch: { score: { $gt: 80 } }, type: 'exam' } }

C. { scores: { $elemMatch: { score: { $gt: 80 } }, type: { $eq: 'exam' } } }

D. { scores: { $elemMatch: { score: { $gt: 80 }, type: 'exam' } } }

Solution

1. Step 1: Understand $elemMatch syntax

   The correct syntax requires an object inside $elemMatch with each condition as a field: score with $gt operator and type with exact match.

2. Step 2: Analyze options

   { scores: { $elemMatch: { $gt: 80, type: 'exam' } } } misuses $gt without a field name. { scores: { $elemMatch: { score: { $gt: 80 } }, type: 'exam' } } incorrectly places type outside $elemMatch. { scores: { $elemMatch: { score: { $gt: 80 } }, type: { $eq: 'exam' } } } also incorrectly places type outside $elemMatch. { scores: { $elemMatch: { score: { $gt: 80 }, type: 'exam' } } } correctly places both conditions inside $elemMatch.

3. Final Answer:

   { scores: { $elemMatch: { score: { $gt: 80 }, type: 'exam' } } } -> Option D

4. Quick Check:

   Both conditions inside $elemMatch object [OK]

Hint: Put all conditions inside one $elemMatch object [OK]

Common Mistakes:

• Placing some conditions outside $elemMatch
• Using $gt without field name
• Misplacing the type condition outside $elemMatch

3. Given the collection documents:

{ _id: 1, grades: [ { score: 85, type: 'exam' }, { score: 70, type: 'quiz' } ] }

{ _id: 2, grades: [ { score: 90, type: 'quiz' }, { score: 75, type: 'exam' } ] }

What documents will this query return?

{ grades: { $elemMatch: { score: { $gt: 80 }, type: 'exam' } } }

medium

A. Only document with _id: 1

B. Only document with _id: 2

C. Both documents with _id: 1 and _id: 2

D. No documents

Solution

1. Step 1: Check document _id: 1

   It has grades with score 85 and type 'exam' which matches score > 80 and type 'exam'. So it matches.

2. Step 2: Check document _id: 2

   Grades are {score: 90, type: 'quiz'} and {score: 75, type: 'exam'}. No single element has both score > 80 and type 'exam' together.

3. Final Answer:

   Only document with _id: 1 -> Option A

4. Quick Check:

   Match requires both conditions on same element [OK]

Hint: Check each array element for all conditions together [OK]

Common Mistakes:

• Matching documents if conditions appear in different elements
• Ignoring the type field condition
• Assuming any element with score > 80 matches

4. You wrote this query to find documents where items array has an element with price less than 20 and qty greater than 5:

{ items: { $elemMatch: { price: { $lt: 20 }, qty: { $gt: 5 } } } }

But it returns no results, even though you know such documents exist. What is the likely problem?

medium

A. You should use $and instead of $elemMatch.

B. The fields price and qty are not in the same array element.

C. The query syntax is invalid and causes an error.

D. MongoDB does not support comparison operators inside $elemMatch.

Solution

1. Step 1: Understand $elemMatch behavior

   $elemMatch requires all conditions to be true on the same array element.

2. Step 2: Analyze the problem

   If price < 20 and qty > 5 exist but in different elements, the query returns no results because no single element satisfies both.

3. Final Answer:

   The fields price and qty are not in the same array element. -> Option B

4. Quick Check:

   All conditions must match one element [OK]

Hint: Check if conditions apply to same array element [OK]

Common Mistakes:

• Assuming $elemMatch matches conditions across elements
• Thinking $and replaces $elemMatch for arrays
• Believing MongoDB disallows operators inside $elemMatch

5. You have a collection of products with a field reviews which is an array of objects like { rating: Number, user: String, verified: Boolean }. You want to find products that have at least one review with rating >= 4, user 'Alice', and verified true. Which query correctly uses $elemMatch to achieve this?

hard

A. { reviews: { $elemMatch: { rating: { $gte: 4 }, user: 'Alice', verified: true } } }

B. { reviews: { rating: { $gte: 4 }, user: 'Alice', verified: true } }

C. { reviews: { $all: [ { rating: { $gte: 4 } }, { user: 'Alice' }, { verified: true } ] } }

D. { reviews: { $elemMatch: { rating: 4, user: 'Alice', verified: true } } }

Solution

1. Step 1: Understand the conditions

   We want one review element where rating is at least 4, user is 'Alice', and verified is true.

2. Step 2: Analyze query options

   { reviews: { $elemMatch: { rating: { $gte: 4 }, user: 'Alice', verified: true } } } correctly uses $elemMatch with all conditions combined, including $gte for rating. { reviews: { rating: { $gte: 4 }, user: 'Alice', verified: true } } misses $elemMatch, so conditions apply to different elements. { reviews: { $all: [ { rating: { $gte: 4 } }, { user: 'Alice' }, { verified: true } ] } } misuses $all which matches elements individually, not combined. { reviews: { $elemMatch: { rating: 4, user: 'Alice', verified: true } } } uses rating: 4 (exact), not >= 4.

3. Final Answer:

   { reviews: { $elemMatch: { rating: { $gte: 4 }, user: 'Alice', verified: true } } } -> Option A

4. Quick Check:

   Use $elemMatch with all conditions and correct operators [OK]

Hint: Combine all conditions inside $elemMatch with correct operators [OK]

Common Mistakes:

• Omitting $elemMatch causing wrong matches
• Using exact match instead of comparison operators
• Using $all which checks elements separately