Array of embedded documents queries in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find documents where the array 'scores' contains an element with 'score' equal to 100.

MongoDB

db.students.find({"scores.score": [1])

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

A100

B"100"

Cscore

D{score: 100}

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string "100" instead of the number 100.

Trying to match the whole embedded document instead of just the score value.

The query matches documents where any element in the 'scores' array has a 'score' field equal to 100. The value should be a number, not a string or object.

2fill in blank

medium

Complete the code to find documents where the 'scores' array contains an element with 'type' equal to 'exam'.

MongoDB

db.students.find({"scores.type": [1])

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

Aexam

B"exam"

C'exam'

D{type: 'exam'}

Attempts:

3 left

💡 Hint

Common Mistakes

Not quoting the string value, causing a syntax error.

Using single quotes without escaping inside JSON.

The value for 'type' is a string, so it must be enclosed in quotes in the query. Using double quotes inside the query is correct.

3fill in blank

hard

Fix the error in the query to find documents where the 'scores' array contains an element with 'score' greater than 90.

MongoDB

db.students.find({"scores.score": [1])

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

A{ $lt: 90 }

B{ gt: 90 }

C{ $gte: 90 }

D{ $gt: 90 }

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the '$' sign before 'gt'.

Using the wrong operator like '$lt' or 'gte' when 'gt' is needed.

The correct MongoDB operator for 'greater than' is '$gt'. It must be prefixed with a dollar sign.

4fill in blank

hard

Fill both blanks to find documents where the 'scores' array contains an element with 'type' equal to 'quiz' and 'score' less than 50.

MongoDB

db.students.find({"scores": { $elemMatch: {"type": [1], "score": [2] } } })

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

A"quiz"

B{ $lt: 50 }

C{ $gt: 50 }

D"exam"

Attempts:

3 left

💡 Hint

Common Mistakes

Not quoting the string 'quiz'.

Using $gt instead of $lt for the score condition.

The $elemMatch operator matches array elements that satisfy both conditions: 'type' is the string "quiz" and 'score' is less than 50 using $lt.

5fill in blank

hard

Fill all three blanks to find documents where the 'scores' array contains an element with 'type' equal to 'homework', 'score' greater than or equal to 80, and 'score' less than 100.

MongoDB

db.students.find({"scores": { $elemMatch: {"type": [1], "score": [2], "score": [3] } } })

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

A"homework"

B{ $gte: 80 }

C{ $lt: 100 }

D"quiz"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same key 'score' twice without $elemMatch (which is invalid).

Not quoting the string 'homework'.

Using wrong operators like $gt instead of $gte.

The $elemMatch operator matches array elements where 'type' is "homework", 'score' is greater than or equal to 80 ($gte), and less than 100 ($lt).

Practice

1.

Which MongoDB query operator is used to match documents where at least one element in an array of embedded documents meets multiple conditions?

easy

A. $exists

B. $all

C. $in

D. $elemMatch

Solution

1. Step 1: Understand the need for multiple conditions on one embedded document

   When querying arrays of embedded documents, multiple conditions on the same element require a special operator.

2. Step 2: Identify the correct operator for matching multiple conditions on one element

   $elemMatch matches documents where at least one array element satisfies all specified conditions.

3. Final Answer:

   $elemMatch -> Option D

4. Quick Check:

   Multiple conditions on one embedded doc = $elemMatch [OK]

Hint: Use $elemMatch for multiple conditions on one array element [OK]

Common Mistakes:

• Using $all which matches all elements separately
• Using $in which matches any value but not multiple conditions
• Confusing $exists with array element matching

2.

Which of the following is the correct MongoDB query syntax to find documents where the comments array contains an embedded document with author equal to "Alice"?

{ comments: { ? } }

easy

A. { $in: { author: "Alice" } }

B. { author: "Alice" }

C. { $elemMatch: { author: "Alice" } }

D. { $all: { author: "Alice" } }

Solution

1. Step 1: Identify simple query for matching one field in embedded documents

   To find documents where any embedded document in an array has a field equal to a value, use $elemMatch or direct matching.

2. Step 2: Recognize that { comments: { $elemMatch: { author: "Alice" } } } matches if any embedded document has author "Alice"

   This syntax matches documents where at least one element in comments array has author "Alice".

3. Final Answer:

   { $elemMatch: { author: "Alice" } } -> Option C

4. Quick Check:

   Use $elemMatch for embedded document match [OK]

Hint: Use $elemMatch for embedded document match [OK]

Common Mistakes:

• Using $in or $all incorrectly with embedded documents
• Confusing array field with embedded document field
• Using direct object without $elemMatch may not always work as expected

3.

Given the collection posts with documents like:

{ _id: 1, comments: [ { author: "Bob", likes: 5 }, { author: "Alice", likes: 3 } ] }

What will the query db.posts.find({ comments: { $elemMatch: { author: "Alice", likes: { $gt: 2 } } } }) return?

medium

A. [{ _id: 1, comments: [...] }]

B. [] (empty array)

C. Syntax error

D. All documents in posts collection

Solution

1. Step 1: Understand $elemMatch with multiple conditions

   The query looks for documents where at least one comment has author "Alice" and likes greater than 2.

2. Step 2: Check the example document for matching embedded document

   The second comment has author "Alice" and likes 3, which is greater than 2, so it matches.

3. Final Answer:

   [{ _id: 1, comments: [...] }] -> Option A

4. Quick Check:

   Matching embedded doc with $elemMatch returns document [OK]

Hint: Check all conditions inside $elemMatch for one embedded doc [OK]

Common Mistakes:

• Assuming $elemMatch matches separate elements for each condition
• Expecting empty result when conditions are met
• Confusing $elemMatch with $all

4.

Identify the error in this MongoDB query to find documents where reviews array has an embedded document with rating greater than 4 and verified true:

db.collection.find({ reviews: { rating: { $gt: 4 }, verified: true } })

medium

A. reviews field should not be queried as an array

B. Missing $elemMatch to combine multiple conditions on one embedded document

C. Incorrect use of $gt operator

D. Syntax error due to missing commas

Solution

1. Step 1: Recognize multiple conditions on one embedded document

   Multiple conditions on fields inside an array element require $elemMatch to ensure they apply to the same element.

2. Step 2: Identify missing $elemMatch usage

   The query incorrectly tries to match multiple fields directly inside the array field, which matches documents with separate elements for each condition, not the same element.

3. Final Answer:

   Missing $elemMatch to combine multiple conditions on one embedded document -> Option B

4. Quick Check:

   Multiple conditions on one element need $elemMatch [OK]

Hint: Use $elemMatch for multiple conditions on same embedded doc [OK]

Common Mistakes:

• Omitting $elemMatch for multiple conditions
• Assuming direct object matches all conditions on one element
• Misunderstanding array query behavior

5.

You have a collection orders with documents like:

{ _id: 1, items: [ { product: "Pen", qty: 10 }, { product: "Notebook", qty: 5 } ] }

Write a query to find orders where items contains a product "Pen" with quantity at least 10, and also a product "Notebook" with quantity at least 5.

Which query correctly finds such orders?

hard

A. { items: { $elemMatch: { product: "Pen", qty: { $gte: 10 } } }, items: { $elemMatch: { product: "Notebook", qty: { $gte: 5 } } } }

B. { items: { $elemMatch: { product: "Pen", qty: { $gte: 10 }, product: "Notebook", qty: { $gte: 5 } } } }

C. { items: { product: "Pen", qty: { $gte: 10 }, product: "Notebook", qty: { $gte: 5 } } }

D. { items: { $in: [ { product: "Pen", qty: { $gte: 10 } }, { product: "Notebook", qty: { $gte: 5 } } ] } }

Solution

1. Step 1: Understand the need to match two different embedded documents in the array

   We want orders where items array has one element matching Pen with qty >= 10 and another element matching Notebook with qty >= 5.

2. Step 2: Use repeated array field with $elemMatch to require both conditions on separate elements

   Repeating the array field key with $elemMatch for each condition ensures at least one array element satisfies each set of conditions.

3. Step 3: Eliminate incorrect options

   { items: { $elemMatch: { product: "Pen", qty: { $gte: 10 }, product: "Notebook", qty: { $gte: 5 } } } } tries to match both products in one element (impossible). { items: { product: "Pen", qty: { $gte: 10 }, product: "Notebook", qty: { $gte: 5 } } } is invalid syntax. { items: { $in: [ { product: "Pen", qty: { $gte: 10 } }, { product: "Notebook", qty: { $gte: 5 } } ] } } uses $in which matches any element but not both conditions.

4. Final Answer:

   { items: { $elemMatch: { product: "Pen", qty: { $gte: 10 } } }, items: { $elemMatch: { product: "Notebook", qty: { $gte: 5 } } } } -> Option A

5. Quick Check:

   Repeated field + $elemMatch for multiple different embedded docs [OK]

Hint: Use repeated array field with $elemMatch for multiple different embedded docs [OK]

Common Mistakes:

• Trying to match multiple products in one embedded document
• Using $elemMatch alone for multiple different elements
• Using $in which matches any but not all conditions