$addFields for computed fields in MongoDB - Mini Project: Build & Apply

$addFields for computed fields in MongoDB

📖 Scenario: You are managing a small online store database. Each product document has a price and quantity field. You want to add a new field that shows the total value of each product's stock (price multiplied by quantity).

🎯 Goal: Build a MongoDB aggregation pipeline that uses $addFields to add a computed field called totalValue to each product document. This field should be the product of price and quantity.

📋 What You'll Learn

Create a collection called products with three documents having exact fields and values.

Add a variable called pipeline that holds an array for the aggregation pipeline.

Use $addFields in the pipeline to add a new field totalValue computed as price * quantity.

Complete the aggregation command using db.products.aggregate(pipeline).

💡 Why This Matters

🌍 Real World

Stores often need to calculate total stock value for inventory management and reporting.

💼 Career

Understanding aggregation pipelines and computed fields is essential for database developers and data analysts working with MongoDB.

1

Create the products collection with sample documents

Create a variable called products that is an array with these three exact documents: { _id: 1, name: 'Pen', price: 1.5, quantity: 100 }, { _id: 2, name: 'Notebook', price: 3, quantity: 200 }, and { _id: 3, name: 'Eraser', price: 0.5, quantity: 300 }.

MongoDB

# Create the products array with three documents
# Your code here

Need a hint?

Use a list of dictionaries with exact keys and values as shown.

2

Create the aggregation pipeline variable

Create a variable called pipeline and set it to an empty array [] to prepare for the aggregation stages.

MongoDB

products = [
    { '_id': 1, 'name': 'Pen', 'price': 1.5, 'quantity': 100 },
    { '_id': 2, 'name': 'Notebook', 'price': 3, 'quantity': 200 },
    { '_id': 3, 'name': 'Eraser', 'price': 0.5, 'quantity': 300 }
]
# Create an empty pipeline array
# Your code here

Need a hint?

Just create a variable named pipeline and assign an empty list [].

3

Add the $addFields stage to compute totalValue

Add a dictionary to the pipeline array that uses $addFields to add a new field called totalValue. This field should be computed as the multiplication of price and quantity using { $multiply: ["$price", "$quantity"] }.

MongoDB

products = [
    { '_id': 1, 'name': 'Pen', 'price': 1.5, 'quantity': 100 },
    { '_id': 2, 'name': 'Notebook', 'price': 3, 'quantity': 200 },
    { '_id': 3, 'name': 'Eraser', 'price': 0.5, 'quantity': 300 }
]
pipeline = []
# Add $addFields stage to pipeline to compute totalValue
# Your code here

Need a hint?

Use { '$addFields': { 'totalValue': { '$multiply': ["$price", "$quantity"] } } } inside the pipeline list.

4

Complete the aggregation command

Write a line that calls db.products.aggregate(pipeline) to run the aggregation pipeline on the products collection.

MongoDB

products = [
    { '_id': 1, 'name': 'Pen', 'price': 1.5, 'quantity': 100 },
    { '_id': 2, 'name': 'Notebook', 'price': 3, 'quantity': 200 },
    { '_id': 3, 'name': 'Eraser', 'price': 0.5, 'quantity': 300 }
]
pipeline = [
    { '$addFields': { 'totalValue': { '$multiply': ["$price", "$quantity"] } } }
]
# Run the aggregation pipeline on the products collection
# Your code here

Need a hint?

Use db.products.aggregate(pipeline) exactly to run the pipeline.