Array of embedded documents queries in MongoDB - Mini Project: Build & Apply

Querying Arrays of Embedded Documents in MongoDB

📖 Scenario: You are managing a small library database in MongoDB. Each book document contains an array of embedded documents representing reviews from readers. Each review has a reviewer name and a rating score.Your task is to write queries that find books based on conditions inside these embedded review documents.

🎯 Goal: Build MongoDB queries that filter books by conditions on the array of embedded reviews documents.

📋 What You'll Learn

Create a collection named books with documents containing an array field reviews of embedded documents

Add a configuration variable to specify a minimum rating threshold

Write a query to find books with at least one review having a rating greater than or equal to the threshold

Write a query to find books where a specific reviewer gave a rating below the threshold

💡 Why This Matters

🌍 Real World

Many real-world databases store related data as arrays of embedded documents, such as product reviews, comments, or order items. Knowing how to query these arrays is essential for retrieving meaningful information.

💼 Career

Database developers and data analysts often need to write queries that filter documents based on nested array data. Mastering $elemMatch and embedded document queries is a key skill for working with MongoDB.

1

Create the books collection with embedded reviews

Create a MongoDB collection named books and insert these two documents exactly:
{ title: "Learn MongoDB", reviews: [{ reviewer: "Alice", rating: 5 }, { reviewer: "Bob", rating: 3 }] }
{ title: "Mastering Databases", reviews: [{ reviewer: "Charlie", rating: 4 }, { reviewer: "Alice", rating: 2 }] }

MongoDB

// Insert the two book documents with embedded reviews
// Your code here

Hint

Use db.books.insertMany() with an array of two objects. Each object must have a title string and a reviews array of embedded documents with reviewer and rating.

2

Set a minimum rating threshold variable

Create a JavaScript variable named minRating and set it to 4 to use as the minimum rating threshold for queries.

MongoDB

db.books.insertMany([
  { title: "Learn MongoDB", reviews: [ { reviewer: "Alice", rating: 5 }, { reviewer: "Bob", rating: 3 } ] },
  { title: "Mastering Databases", reviews: [ { reviewer: "Charlie", rating: 4 }, { reviewer: "Alice", rating: 2 } ] }
])

// Create a variable minRating and set it to 4
// Your code here

Hint

Use const minRating = 4 to create the variable.

3

Query books with any review rating >= minRating

Write a MongoDB query named highRatedBooks that finds all documents in books where the reviews array contains at least one embedded document with rating greater than or equal to minRating. Use the $elemMatch operator.

MongoDB

db.books.insertMany([
  { title: "Learn MongoDB", reviews: [ { reviewer: "Alice", rating: 5 }, { reviewer: "Bob", rating: 3 } ] },
  { title: "Mastering Databases", reviews: [ { reviewer: "Charlie", rating: 4 }, { reviewer: "Alice", rating: 2 } ] }
])

const minRating = 4

// Write a query highRatedBooks to find books with any review rating >= minRating
// Your code here

Hint

Use db.books.find({ reviews: { $elemMatch: { rating: { $gte: minRating } } } }) and assign it to highRatedBooks.

4

Query books where Alice gave a rating below minRating

Write a MongoDB query named aliceLowRatings that finds all documents in books where the reviews array contains an embedded document with reviewer equal to "Alice" and rating less than minRating. Use the $elemMatch operator.

MongoDB

db.books.insertMany([
  { title: "Learn MongoDB", reviews: [ { reviewer: "Alice", rating: 5 }, { reviewer: "Bob", rating: 3 } ] },
  { title: "Mastering Databases", reviews: [ { reviewer: "Charlie", rating: 4 }, { reviewer: "Alice", rating: 2 } ] }
])

const minRating = 4

const highRatedBooks = db.books.find({ reviews: { $elemMatch: { rating: { $gte: minRating } } } })

// Write a query aliceLowRatings to find books where Alice's rating is less than minRating
// Your code here

Hint

Use db.books.find({ reviews: { $elemMatch: { reviewer: "Alice", rating: { $lt: minRating } } } }) and assign it to aliceLowRatings.

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