$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

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

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

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

Hint

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

Practice

1. What is the primary purpose of the $addFields stage in a MongoDB aggregation pipeline?

easy

A. To add new fields or update existing fields with computed values in documents

B. To delete fields from documents permanently

C. To sort documents based on a field

D. To group documents by a specific field

Solution

1. Step 1: Understand the role of $addFields

   $addFields is used to add new fields or update existing fields with computed values inside documents during aggregation.

2. Step 2: Compare with other stages

   Deleting fields is done by $project, sorting by $sort, and grouping by $group. These are different from $addFields.

3. Final Answer:

   To add new fields or update existing fields with computed values in documents -> Option A

4. Quick Check:

   $addFields adds or updates fields [OK]

Hint: Remember: $addFields enriches documents with new computed fields [OK]

Common Mistakes:

• Confusing $addFields with $project which removes fields
• Thinking $addFields sorts or groups documents
• Assuming $addFields deletes fields

2. Which of the following is the correct syntax to add a new field named totalPrice that multiplies price and quantity using $addFields?

easy

A. { $addFields: { totalPrice: "$price" * "$quantity" } }

B. { $addFields: { totalPrice: "$price" + "$quantity" } }

C. { $addFields: { totalPrice: { $multiply: ["$price", "$quantity"] } } }

D. { $addFields: { totalPrice: { $add: ["$price", "$quantity"] } } }

Solution

1. Step 1: Identify correct operator for multiplication

   MongoDB uses $multiply with an array of fields to multiply values.

2. Step 2: Check syntax correctness

   { $addFields: { totalPrice: { $multiply: ["$price", "$quantity"] } } } correctly uses $multiply with array syntax and field references with $.

3. Final Answer:

   { $addFields: { totalPrice: { $multiply: ["$price", "$quantity"] } } } -> Option C

4. Quick Check:

   Use $multiply with array for multiplication [OK]

Hint: Use $multiply with array of fields for multiplication [OK]

Common Mistakes:

• Using arithmetic operators like * directly inside $addFields
• Confusing $add with multiplication
• Missing $ before field names

3. Given the collection documents:

{ "item": "apple", "price": 2, "quantity": 5 }

What will be the output document after applying this aggregation stage?

{ $addFields: { totalCost: { $multiply: ["$price", "$quantity"] } } }

medium

A. Error: Invalid operator

B. { "item": "apple", "price": 2, "quantity": 5, "totalCost": "2*5" }

C. { "item": "apple", "price": 2, "quantity": 5 }

D. { "item": "apple", "price": 2, "quantity": 5, "totalCost": 10 }

Solution

1. Step 1: Calculate the multiplication of price and quantity

   2 multiplied by 5 equals 10.

2. Step 2: Understand $addFields adds the computed field

   The new field totalCost will be added with value 10.

3. Final Answer:

   { "item": "apple", "price": 2, "quantity": 5, "totalCost": 10 } -> Option D

4. Quick Check:

   2 * 5 = 10 added as totalCost [OK]

Hint: Multiply fields inside $addFields to get computed values [OK]

Common Mistakes:

• Expecting string expression instead of computed number
• Forgetting to add the new field in output
• Assuming syntax error when operator is correct

4. Identify the error in this aggregation stage:

{ $addFields: { discountPrice: { $multiply: "$price", 0.9 } } }

medium

A. Cannot multiply a field by a number in $addFields

B. Incorrect use of $multiply operator syntax; expects an array

C. Field name discountPrice is invalid

D. Missing $ before price

Solution

1. Step 1: Check $multiply syntax

   $multiply requires an array of expressions, but here it is given two separate arguments.

2. Step 2: Identify correct syntax

   The correct syntax is { $multiply: ["$price", 0.9] } using an array.

3. Final Answer:

   Incorrect use of $multiply operator syntax; expects an array -> Option B

4. Quick Check:

   $multiply needs array of operands [OK]

Hint: Always use array syntax with $multiply [OK]

Common Mistakes:

• Passing arguments to $multiply without array brackets
• Assuming field names cannot be multiplied by numbers
• Confusing field name syntax with missing $

5. You have documents with fields price and taxRate. You want to add a field finalPrice that is price plus price multiplied by taxRate. Which $addFields stage correctly computes this?

hard

A. { $addFields: { finalPrice: { $add: ["$price", { $multiply: ["$price", "$taxRate"] }] } } }

B. { $addFields: { finalPrice: "$price" + "$price" * "$taxRate" } }

C. { $addFields: { finalPrice: { $multiply: ["$price", "$taxRate"] } } }

D. { $addFields: { finalPrice: { $add: ["$price", "$taxRate"] } } }

Solution

1. Step 1: Understand the formula for finalPrice

   The formula is price + (price * taxRate), which requires addition and multiplication.

2. Step 2: Check correct MongoDB operators usage

   { $addFields: { finalPrice: { $add: ["$price", { $multiply: ["$price", "$taxRate"] }] } } } uses $add with an array containing price and the multiplication result of price and taxRate, which is correct syntax.

3. Final Answer:

   { $addFields: { finalPrice: { $add: ["$price", { $multiply: ["$price", "$taxRate"] }] } } } -> Option A

4. Quick Check:

   Use nested $add and $multiply for combined calculations [OK]

Hint: Nest $multiply inside $add for combined calculations [OK]

Common Mistakes:

• Trying to use arithmetic operators directly inside $addFields
• Adding price and taxRate directly without multiplication
• Using wrong operator or missing array brackets