$push accumulator for building arrays in MongoDB - Mini Project: Build & Apply

$push accumulator for building arrays in MongoDB

📖 Scenario: You are managing a small online bookstore database. Each book document stores information about the book and a list of reviews from customers. You want to group all reviews by book title to see all feedback in one place.

🎯 Goal: Build an aggregation query using the $push accumulator to collect all reviews for each book title into an array.

📋 What You'll Learn

Create a collection named books with documents containing title and review fields.

Define a variable groupStage for the aggregation $group stage.

Use the $push accumulator inside groupStage to collect reviews into an array for each book title.

Write the complete aggregation pipeline using db.books.aggregate() with the groupStage.

💡 Why This Matters

🌍 Real World

Collecting and grouping user reviews or comments by product or article is common in many applications like e-commerce or blogs.

💼 Career

Understanding aggregation with $push is essential for data analysis and reporting tasks in MongoDB database roles.

1

Create the books collection with sample documents

Create a variable called books and assign it an array with these exact documents: { title: "Book A", review: "Great read" }, { title: "Book B", review: "Informative" }, { title: "Book A", review: "Loved it" }

MongoDB

const books = [
  // Your code here
];

Need a hint?

Use an array of objects with exact title and review fields as shown.

2

Define the groupStage for aggregation

Create a variable called groupStage and assign it an object with a $group key. Group by _id: "$title" and prepare to use $push to collect reviews.

MongoDB

const books = [
  { title: "Book A", review: "Great read" },
  { title: "Book B", review: "Informative" },
  { title: "Book A", review: "Loved it" }
];

const groupStage = {
  // Your code here
};

Need a hint?

Use $group with _id set to "$title" and reviews using $push on "$review".

3

Write the aggregation pipeline using groupStage

Create a variable called pipeline and assign it an array containing the groupStage object.

MongoDB

const books = [
  { title: "Book A", review: "Great read" },
  { title: "Book B", review: "Informative" },
  { title: "Book A", review: "Loved it" }
];

const groupStage = {
  $group: {
    _id: "$title",
    reviews: { $push: "$review" }
  }
};

const pipeline = [
  // Your code here
];

Need a hint?

Put the groupStage object inside an array assigned to pipeline.

4

Complete the aggregation query using db.books.aggregate()

Write a line of code that calls db.books.aggregate(pipeline) and assign the result to a variable called result.

MongoDB

const books = [
  { title: "Book A", review: "Great read" },
  { title: "Book B", review: "Informative" },
  { title: "Book A", review: "Loved it" }
];

const groupStage = {
  $group: {
    _id: "$title",
    reviews: { $push: "$review" }
  }
};

const pipeline = [
  groupStage
];

// Your code here

Need a hint?

Call db.books.aggregate(pipeline) and assign it to result.