$first and $last accumulators in MongoDB - Mini Project: Build & Apply

$first and $last Accumulators in MongoDB Aggregation

📖 Scenario: You work at a bookstore that tracks sales data in a MongoDB collection. Each sale record includes the book title, the date of sale, and the price. You want to analyze the sales to find the first and last sale price for each book.

🎯 Goal: Build a MongoDB aggregation pipeline that groups sales by book title and uses the $first and $last accumulators to find the first and last sale price for each book.

📋 What You'll Learn

Create a collection called sales with documents containing title, date, and price fields.

Add a variable sortOrder to specify sorting by date ascending.

Write an aggregation pipeline that groups sales by title and uses $first and $last to get the first and last price per book.

Complete the aggregation pipeline by adding the $sort stage to order results by title.

💡 Why This Matters

🌍 Real World

Bookstores and retailers often analyze sales data to understand price changes over time for products.

💼 Career

Knowing how to use MongoDB aggregation with $first and $last is useful for data analysts and backend developers working with time-series or ordered data.

1

Create the sales collection with sample data

Create a variable called sales and assign it an array of documents with these exact entries: { title: "Book A", date: ISODate("2023-01-01"), price: 10 }, { title: "Book A", date: ISODate("2023-01-05"), price: 12 }, { title: "Book B", date: ISODate("2023-01-02"), price: 15 }, { title: "Book B", date: ISODate("2023-01-10"), price: 18 }

MongoDB

const sales = [
  // Your documents here
]

Need a hint?

Use new Date("YYYY-MM-DD") to create date objects in JavaScript.

2

Add a sortOrder variable to sort by date ascending

Create a variable called sortOrder and assign it an object that sorts by date in ascending order using { date: 1 }.

MongoDB

const sales = [
  { title: "Book A", date: new Date("2023-01-01"), price: 10 },
  { title: "Book A", date: new Date("2023-01-05"), price: 12 },
  { title: "Book B", date: new Date("2023-01-02"), price: 15 },
  { title: "Book B", date: new Date("2023-01-10"), price: 18 }
]

// Your code here

Need a hint?

Sorting by date ascending means using 1 as the value.

3

Write an aggregation pipeline using $first and $last

Create a variable called pipeline and assign it an array with these stages: first a { $sort: sortOrder } stage, then a { $group } stage that groups by title and uses $first to get the first price as firstPrice and $last to get the last price as lastPrice.

MongoDB

const sales = [
  { title: "Book A", date: new Date("2023-01-01"), price: 10 },
  { title: "Book A", date: new Date("2023-01-05"), price: 12 },
  { title: "Book B", date: new Date("2023-01-02"), price: 15 },
  { title: "Book B", date: new Date("2023-01-10"), price: 18 }
]

const sortOrder = { date: 1 }

// Your code here

Need a hint?

Remember to sort before grouping to get correct $first and $last values.

4

Add a $sort stage to order results by title

Add a final stage to the pipeline array that sorts the grouped results by _id in ascending order using { $sort: { _id: 1 } }.

MongoDB

const sales = [
  { title: "Book A", date: new Date("2023-01-01"), price: 10 },
  { title: "Book A", date: new Date("2023-01-05"), price: 12 },
  { title: "Book B", date: new Date("2023-01-02"), price: 15 },
  { title: "Book B", date: new Date("2023-01-10"), price: 18 }
]

const sortOrder = { date: 1 }

const pipeline = [
  { $sort: sortOrder },
  { $group: {
      _id: "$title",
      firstPrice: { $first: "$price" },
      lastPrice: { $last: "$price" }
    }
  }
]

// Your code here

Need a hint?

Sorting by _id ascending orders the grouped results by book title.