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

Calculate Average Scores Using $avg Accumulator in MongoDB

📖 Scenario: You are managing a small school's student test scores stored in a MongoDB collection. You want to find the average score of all students to understand overall performance.

🎯 Goal: Build a MongoDB aggregation query that uses the $avg accumulator to calculate the average score of all students in the collection.

📋 What You'll Learn

Create a collection named students with documents containing name and score fields.

Add a configuration variable scoreField to hold the field name score.

Write an aggregation pipeline that uses $group stage with $avg accumulator on the scoreField.

Complete the aggregation query to output the average score with the field name averageScore.

💡 Why This Matters

🌍 Real World

Calculating averages is common in reporting systems, such as finding average sales, test scores, or ratings stored in databases.

💼 Career

Understanding how to use MongoDB aggregation and accumulators like <code>$avg</code> is essential for data analysts and backend developers working with NoSQL databases.

1

Create the students collection with sample data

Create a MongoDB collection called students with these exact documents: { name: "Alice", score: 85 }, { name: "Bob", score: 90 }, and { name: "Charlie", score: 78 }.

MongoDB

// Insert the documents into the students collection
// Your code here

Need a hint?

Use db.students.insertMany([...]) to add multiple documents at once.

2

Add a configuration variable for the score field

Create a variable called scoreField and set it to the string "score" to represent the field name holding the scores.

MongoDB

db.students.insertMany([
  { name: "Alice", score: 85 },
  { name: "Bob", score: 90 },
  { name: "Charlie", score: 78 }
])

// Create the scoreField variable
// Your code here

Need a hint?

Use const scoreField = "score" to store the field name.

3

Write the aggregation pipeline using $avg

Write a MongoDB aggregation pipeline called pipeline that uses a $group stage to calculate the average of the field stored in scoreField. Use $avg: "$" + scoreField to reference the field dynamically. Name the output field averageScore.

MongoDB

db.students.insertMany([
  { name: "Alice", score: 85 },
  { name: "Bob", score: 90 },
  { name: "Charlie", score: 78 }
])

const scoreField = "score";

// Create the aggregation pipeline to calculate average score
// Your code here

Need a hint?

Use $group with _id: null to aggregate all documents together.

4

Complete the aggregation query to get the average score

Run the aggregation query on the students collection using the pipeline variable and assign the result to a variable called result.

MongoDB

db.students.insertMany([
  { name: "Alice", score: 85 },
  { name: "Bob", score: 90 },
  { name: "Charlie", score: 78 }
])

const scoreField = "score";

const pipeline = [
  {
    $group: {
      _id: null,
      averageScore: { $avg: "$" + scoreField }
    }
  }
];

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

Need a hint?

Use db.students.aggregate(pipeline) to run the aggregation.