Array expressions ($size, $arrayElemAt, $filter) in MongoDB - Mini Project: Build & Apply

Working with Array Expressions in MongoDB: $size, $arrayElemAt, $filter

📖 Scenario: You are managing a small online bookstore database. Each book document contains an array of reviews from readers. You want to analyze these reviews to understand how many reviews each book has, pick a specific review, and filter reviews based on rating.

🎯 Goal: Build a MongoDB aggregation pipeline that uses $size to count reviews, $arrayElemAt to pick a specific review, and $filter to select reviews with a rating above a threshold.

📋 What You'll Learn

Create a collection called books with documents containing a title and an array field reviews.

Add a variable minRating to set the minimum rating for filtering reviews.

Write an aggregation pipeline that adds fields: reviewCount using $size, firstReview using $arrayElemAt, and goodReviews using $filter with minRating.

Complete the pipeline by projecting the new fields along with the book title.

💡 Why This Matters

🌍 Real World

Analyzing customer reviews in an online bookstore to understand feedback quantity and quality.

💼 Career

Using MongoDB aggregation with array expressions is common in data analysis and backend development roles.

1

Create the books collection with sample data

Create a MongoDB collection called books with two documents. Each document must have a title string and a reviews array with exactly these objects: { reviewer: "Alice", rating: 5 }, { reviewer: "Bob", rating: 3 }, and { reviewer: "Charlie", rating: 4 } for the first book, and { reviewer: "Dave", rating: 2 }, { reviewer: "Eve", rating: 5 } for the second book.

MongoDB

// Insert the two book documents with the exact titles and reviews arrays
// Your code here

Need a hint?

Use db.books.insertMany([...]) with the exact documents as described.

2

Set the minimum rating threshold

Create a variable called minRating and set it to the number 4. This will be used to filter reviews with ratings 4 or higher.

MongoDB

db.books.insertMany([...])

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

Need a hint?

Use const minRating = 4 to define the threshold.

3

Build the aggregation pipeline with $size, $arrayElemAt, and $filter

Write an aggregation pipeline on books that adds three new fields: reviewCount using $size on reviews, firstReview using $arrayElemAt to get the first review (index 0), and goodReviews using $filter to include only reviews with rating greater than or equal to minRating. Use $addFields stage.

MongoDB

db.books.insertMany([...])

const minRating = 4

// Write an aggregation pipeline with $addFields to add reviewCount, firstReview, and goodReviews
// Your code here

Need a hint?

Use $addFields with $size, $arrayElemAt, and $filter as shown.

4

Project the final fields including the title

Complete the aggregation pipeline by adding a $project stage that includes title, reviewCount, firstReview, and goodReviews fields only.

MongoDB

db.books.insertMany([...])

const minRating = 4

db.books.aggregate([
  {
    $addFields: {
      reviewCount: { $size: "$reviews" },
      firstReview: { $arrayElemAt: ["$reviews", 0] },
      goodReviews: {
        $filter: {
          input: "$reviews",
          as: "review",
          cond: { $gte: ["$$review.rating", minRating] }
        }
      }
    }
  },
  // Add a $project stage here to include title, reviewCount, firstReview, goodReviews
  // Your code here
])

Need a hint?

Use $project to include only the specified fields with value 1.