Querying array elements directly in MongoDB - Mini Project: Build & Apply

Querying Array Elements Directly in MongoDB

📖 Scenario: You work at a bookstore that stores book information in a MongoDB collection. Each book document has a title and an array of reviews. Each review has a reviewer's name and a rating.You want to find books that have at least one review with a rating of 5.

🎯 Goal: Build a MongoDB query that finds books with at least one review having a rating of 5 by querying array elements directly.

📋 What You'll Learn

Create a collection named books with documents containing title and reviews array

Add a configuration variable targetRating set to 5

Write a query that finds books where any review's rating equals targetRating

Complete the query to return only the title and matching reviews elements

💡 Why This Matters

🌍 Real World

Many applications store lists of related data inside documents. Querying array elements directly helps find specific items quickly.

💼 Career

Database developers and backend engineers often write queries to filter nested array data in MongoDB for efficient data retrieval.

1

Create the books collection with sample data

Create a MongoDB collection called books and insert these exact documents: { title: "Book A", reviews: [{ reviewer: "Alice", rating: 4 }, { reviewer: "Bob", rating: 5 }] } and { title: "Book B", reviews: [{ reviewer: "Carol", rating: 3 }, { reviewer: "Dave", rating: 2 }] }.

MongoDB

// Insert the two book documents into the books collection
// Your code here

Hint

Use db.books.insertMany() with an array of two objects as shown.

2

Set the targetRating variable

Create a variable called targetRating and set it to the number 5.

MongoDB

db.books.insertMany([
  { title: "Book A", reviews: [{ reviewer: "Alice", rating: 4 }, { reviewer: "Bob", rating: 5 }] },
  { title: "Book B", reviews: [{ reviewer: "Carol", rating: 3 }, { reviewer: "Dave", rating: 2 }] }
])

// Create targetRating variable and set it to 5
// Your code here

Hint

Use const targetRating = 5 to create the variable.

3

Write the query to find books with a review rating equal to targetRating

Write a MongoDB query using db.books.find() that finds documents where the reviews array has at least one element with rating equal to targetRating. Use the query { "reviews.rating": targetRating }.

MongoDB

db.books.insertMany([
  { title: "Book A", reviews: [{ reviewer: "Alice", rating: 4 }, { reviewer: "Bob", rating: 5 }] },
  { title: "Book B", reviews: [{ reviewer: "Carol", rating: 3 }, { reviewer: "Dave", rating: 2 }] }
])

const targetRating = 5

// Write the query to find books with reviews.rating equal to targetRating
// Your code here

Hint

Use dot notation "reviews.rating" in the query object.

4

Complete the query to return only title and matching reviews elements

Complete the query by calling db.books.find() with the query and a projection that returns only title and the matching reviews.$ element.

MongoDB

db.books.insertMany([
  { title: "Book A", reviews: [{ reviewer: "Alice", rating: 4 }, { reviewer: "Bob", rating: 5 }] },
  { title: "Book B", reviews: [{ reviewer: "Carol", rating: 3 }, { reviewer: "Dave", rating: 2 }] }
])

const targetRating = 5

const query = { "reviews.rating": targetRating }

// Complete the find() call with projection to return title and matching reviews element
// Your code here

Hint

Use { title: 1, "reviews.$": 1 } as the projection to get only the title and the matched review.

Practice

1. What does the MongoDB query { tags: "mongodb" } do when applied to a collection where tags is an array field?

easy

A. Finds documents where the tags array contains the value "mongodb".

B. Finds documents where the tags array is exactly equal to "mongodb".

C. Finds documents where the tags array is empty.

D. Finds documents where the tags field does not exist.

Solution

1. Step 1: Understand array field querying in MongoDB

   When querying an array field with a value, MongoDB checks if the array contains that value anywhere.

2. Step 2: Analyze the query { tags: "mongodb" }

   This query looks for documents where the tags array includes the string "mongodb" as one of its elements.

3. Final Answer:

   Finds documents where the tags array contains the value "mongodb". -> Option A

4. Quick Check:

   Querying array with value checks for presence = B [OK]

Hint: Querying array with value checks for presence [OK]

Common Mistakes:

• Thinking it matches exact array equality
• Assuming it matches empty arrays
• Confusing missing field with array content

2. Which of the following is the correct MongoDB query syntax to find documents where the array field scores contains the number 85?

easy

A. { scores: { $contains: 85 } }

B. { scores: { $in: 85 } }

C. { scores: 85 }

D. { scores: { $eq: [85] } }

Solution

1. Step 1: Recall MongoDB syntax for matching array elements

   To find documents where an array contains a value, simply use { field: value } syntax.

2. Step 2: Evaluate each option

   { scores: { $contains: 85 } } uses a non-existent operator $contains. { scores: { $eq: [85] } } incorrectly uses $eq with an array. { scores: { $in: 85 } } uses $in incorrectly without an array. { scores: 85 } correctly uses { scores: 85 }.

3. Final Answer:

   { scores: 85 } -> Option C

4. Quick Check:

   Simple value match syntax = D [OK]

Hint: Use {field: value} to match array elements directly [OK]

Common Mistakes:

• Using non-existent operators like $contains
• Misusing $eq with arrays
• Passing non-array to $in operator

3. Given the collection documents:

{ _id: 1, scores: [70, 85, 90] }

{ _id: 2, scores: [60, 75] }

{ _id: 3, scores: [85, 95] }

What will be the result of the query { scores: 85 }?

medium

A. [] (empty array)

B. [{ _id: 2, scores: [60, 75] }]

C. Syntax error

D. [{ _id: 1, scores: [70, 85, 90] }, { _id: 3, scores: [85, 95] }]

Solution

1. Step 1: Identify which documents have 85 in their scores array

   Document 1 has scores [70, 85, 90] which includes 85. Document 3 has scores [85, 95] which also includes 85. Document 2 does not have 85.

2. Step 2: Understand the query result

   The query { scores: 85 } returns all documents where the scores array contains 85, so documents 1 and 3.

3. Final Answer:

   [{ _id: 1, scores: [70, 85, 90] }, { _id: 3, scores: [85, 95] }] -> Option D

4. Quick Check:

   Documents with 85 in scores = C [OK]

Hint: Query returns docs where array contains value [OK]

Common Mistakes:

• Expecting only one document
• Thinking query returns empty if multiple matches
• Confusing syntax error with valid query

4. Consider this incorrect MongoDB query to find documents where the tags array contains both "red" and "blue":

{ tags: { $all: "red", "blue" } }

What is the main issue with this query?

medium

A. The $all operator requires an array of values, not separate arguments.

B. The query should use $elemMatch instead of $all.

C. The field name should be inside quotes.

D. The query is missing a $and operator.

Solution

1. Step 1: Understand $all operator syntax

   The $all operator expects an array of values to match all elements inside the array field.

2. Step 2: Identify the syntax error in the query

   The query incorrectly passes separate arguments to $all instead of an array. Correct syntax is { tags: { $all: ["red", "blue"] } }.

3. Final Answer:

   The $all operator requires an array of values, not separate arguments. -> Option A

4. Quick Check:

   $all needs array syntax = A [OK]

Hint: Use array syntax with $all operator [OK]

Common Mistakes:

• Passing multiple values without array brackets
• Confusing $all with $elemMatch
• Ignoring syntax errors in operator usage

5. You want to find documents where the ratings array contains at least one element greater than 4 and less than 7. Which query correctly uses $elemMatch to achieve this?

hard

A. { ratings: { $gt: 4, $lt: 7 } }

B. { ratings: { $elemMatch: { $gt: 4, $lt: 7 } } }

C. { ratings: { $in: [5, 6] } }

D. { ratings: { $elemMatch: { $gte: 4, $lte: 7 } } }

Solution

1. Step 1: Understand $elemMatch usage for multiple conditions on array elements

   $elemMatch allows specifying multiple conditions that must be true for the same array element.

2. Step 2: Analyze each option for correctness

   { ratings: { $elemMatch: { $gt: 4, $lt: 7 } } } correctly uses $elemMatch with $gt and $lt to find elements >4 and <7. { ratings: { $gt: 4, $lt: 7 } } is invalid syntax because $gt and $lt cannot be used directly on the array field. { ratings: { $in: [5, 6] } } matches specific values but does not cover the range condition. { ratings: { $elemMatch: { $gte: 4, $lte: 7 } } } uses $gte and $lte which includes 4 and 7, not strictly greater and less.

3. Final Answer:

   { ratings: { $elemMatch: { $gt: 4, $lt: 7 } } } -> Option B

4. Quick Check:

   Use $elemMatch for multiple conditions on one element = A [OK]

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

Common Mistakes:

• Using $gt and $lt directly on array field
• Using $in instead of range operators
• Confusing inclusive and exclusive range operators