$sum accumulator in MongoDB - Mini Project: Build & Apply

Calculate Total Sales Using $sum Accumulator in MongoDB

📖 Scenario: You work at a small online store. You have a collection of sales records. Each record shows the product name and the number of units sold. You want to find out the total units sold for all products combined.

🎯 Goal: Build a MongoDB aggregation query that uses the $sum accumulator to calculate the total units sold across all sales records.

📋 What You'll Learn

Create a collection called sales with documents containing product and units fields.

Add a variable to hold the aggregation pipeline.

Use the $group stage with _id: null and totalUnits field using $sum on units.

Complete the aggregation query to return the total units sold.

💡 Why This Matters

🌍 Real World

Stores and businesses often need to calculate total sales or quantities from many records to understand performance.

💼 Career

Knowing how to use MongoDB aggregation and the $sum accumulator is essential for data analysis roles and backend development involving NoSQL databases.

1

Create the sales collection with sample data

Create a variable called sales and assign it an array of documents with these exact entries: { product: "Pen", units: 10 }, { product: "Notebook", units: 5 }, { product: "Eraser", units: 3 }.

MongoDB

// Create the sales array with the given documents
// Your code here

Need a hint?

Use an array of objects with product and units fields exactly as shown.

2

Define the aggregation pipeline variable

Create a variable called pipeline and assign it an empty array to start building the aggregation pipeline.

MongoDB

const sales = [
  { product: "Pen", units: 10 },
  { product: "Notebook", units: 5 },
  { product: "Eraser", units: 3 }
];

// Create an empty pipeline array
// Your code here

Need a hint?

Just create an empty array called pipeline to hold aggregation stages.

3

Add a $group stage using $sum accumulator

Add an object to the pipeline array with a $group stage. Use _id: null and create a field called totalUnits that uses $sum to add all units values from the sales documents.

MongoDB

const sales = [
  { product: "Pen", units: 10 },
  { product: "Notebook", units: 5 },
  { product: "Eraser", units: 3 }
];

const pipeline = [];

// Add $group stage with $sum accumulator
// Your code here

Need a hint?

Use $group with _id: null to aggregate all documents into one group. Use $sum: "$units" to add the units.

4

Complete the aggregation query

Create a variable called totalSales and assign it the result of calling sales.aggregate(pipeline) to run the aggregation pipeline and get the total units sold.

MongoDB

const sales = [
  { product: "Pen", units: 10 },
  { product: "Notebook", units: 5 },
  { product: "Eraser", units: 3 }
];

const pipeline = [
  {
    $group: {
      _id: null,
      totalUnits: { $sum: "$units" }
    }
  }
];

// Run the aggregation and store the result in totalSales
// Your code here

Need a hint?

Use sales.aggregate(pipeline) to run the aggregation pipeline on the sales array.