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

Need a 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

Need a 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

Need a 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

Need a hint?

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