MongoDB Aggregation $addFields

Introduction

The $addFields stage in MongoDB's aggregation framework is used to add new fields to documents or modify existing fields within a pipeline. This stage is useful for transforming your data by calculating new values or altering the structure of your documents before they are passed to subsequent stages in the aggregation pipeline.

Prerequisites

• MongoDB installed and running on your machine.

• Access to mongosh and a populated database with sample data.

Step 1: Start mongosh

1. Open Terminal/Command Prompt:

   • Launch your terminal (macOS/Linux) or command prompt (Windows).

2. Start mongosh:

   • Type the following command and press Enter:

     mongosh

Step 2: Switch to the Database

Switch to the database where your collection is located.

1. Use a Database:

   use myNewDatabase

Step 3: Sample Data

Let’s assume you have a collection named sales with documents that look like this:

{ "_id": 1, "item": "apple", "quantity": 5, "price": 2 }
{ "_id": 2, "item": "banana", "quantity": 10, "price": 1 }
{ "_id": 3, "item": "carrot", "quantity": 7, "price": 1.5 }
{ "_id": 4, "item": "orange", "quantity": 3, "price": 3 }
{ "_id": 5, "item": "broccoli", "quantity": 4, "price": 2.5 }

Step 4: Using $addFields

1. Basic Usage of $addFields

To add a new field to each document, use the $addFields stage. For example, if you want to add a totalPrice field that calculates the total price for each item based on quantity and price:

db.sales.aggregate([
  {
    $addFields: {
      totalPrice: { $multiply: ["$quantity", "$price"] } // Calculate totalPrice
    }
  }
])

Output:

{ "_id": 1, "item": "apple", "quantity": 5, "price": 2, "totalPrice": 10 }
{ "_id": 2, "item": "banana", "quantity": 10, "price": 1, "totalPrice": 10 }
{ "_id": 3, "item": "carrot", "quantity": 7, "price": 1.5, "totalPrice": 10.5 }
{ "_id": 4, "item": "orange", "quantity": 3, "price": 3, "totalPrice": 9 }
{ "_id": 5, "item": "broccoli", "quantity": 4, "price": 2.5, "totalPrice": 10 }

2. Modifying Existing Fields

You can also use $addFields to modify existing fields. For instance, if you want to increase the price of each item by 10%:

db.sales.aggregate([
  {
    $addFields: {
      price: { $multiply: ["$price", 1.1] } // Increase price by 10%
    }
  }
])

Output:

{ "_id": 1, "item": "apple", "quantity": 5, "price": 2.2 }
{ "_id": 2, "item": "banana", "quantity": 10, "price": 1.1 }
{ "_id": 3, "item": "carrot", "quantity": 7, "price": 1.65 }
{ "_id": 4, "item": "orange", "quantity": 3, "price": 3.3 }
{ "_id": 5, "item": "broccoli", "quantity": 4, "price": 2.75 }

3. Adding Multiple Fields

You can add multiple fields in a single $addFields stage. For example, to add both totalPrice and a new field called status based on the quantity:

db.sales.aggregate([
  {
    $addFields: {
      totalPrice: { $multiply: ["$quantity", "$price"] },
      status: { $cond: { if: { $gte: ["$quantity", 5] }, then: "In Stock", else: "Low Stock" } }
    }
  }
])

Output:

{ "_id": 1, "item": "apple", "quantity": 5, "price": 2, "totalPrice": 10, "status": "In Stock" }
{ "_id": 2, "item": "banana", "quantity": 10, "price": 1, "totalPrice": 10, "status": "In Stock" }
{ "_id": 3, "item": "carrot", "quantity": 7, "price": 1.5, "totalPrice": 10.5, "status": "In Stock" }
{ "_id": 4, "item": "orange", "quantity": 3, "price": 3, "totalPrice": 9, "status": "Low Stock" }
{ "_id": 5, "item": "broccoli", "quantity": 4, "price": 2.5, "totalPrice": 10, "status": "Low Stock" }

4. Combining $addFields with Other Stages

You can combine $addFields with other aggregation stages for more complex data transformations. For example, you can first filter items that are in stock and then add the totalPrice:

db.sales.aggregate([
  { $match: { quantity: { $gt: 0 } } }, // Filter for items in stock
  {
    $addFields: {
      totalPrice: { $multiply: ["$quantity", "$price"] }
    }
  }
])

Output:

{ "_id": 1, "item": "apple", "quantity": 5, "price": 2, "totalPrice": 10 }
{ "_id": 2, "item": "banana", "quantity": 10, "price": 1, "totalPrice": 10 }
{ "_id": 3, "item": "carrot", "quantity": 7, "price": 1.5, "totalPrice": 10.5 }
{ "_id": 4, "item": "orange", "quantity": 3, "price": 3, "totalPrice": 9 }
{ "_id": 5, "item": "broccoli", "quantity": 4, "price": 2.5, "totalPrice": 10 }

Conclusion

You have learned how to use the $addFields stage in MongoDB's aggregation framework to add or modify fields in documents. This stage is essential for transforming your data, allowing you to perform calculations and create a more informative dataset for analysis and reporting.

Feel free to reach out if you have any questions or need further guidance!

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