$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

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

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

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

Hint

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

Practice

1. What does the $sum accumulator do in a MongoDB aggregation pipeline?

easy

A. Adds values together or counts documents within groups

B. Finds the average value of a field

C. Sorts documents in ascending order

D. Filters documents based on a condition

Solution

1. Step 1: Understand the role of $sum

   The $sum accumulator adds numeric values or counts documents when used in a $group stage.

2. Step 2: Compare with other accumulators

   Unlike $avg which calculates averages, $sum only totals values or counts documents.

3. Final Answer:

   Adds values together or counts documents within groups -> Option A

4. Quick Check:

   $sum = Adds or counts [OK]

Hint: Remember: $sum totals numbers or counts docs [OK]

Common Mistakes:

• Confusing $sum with $avg
• Thinking $sum sorts data
• Assuming $sum filters documents

2. Which of the following is the correct syntax to sum the field price in a $group stage?

easy

A. { $group: { _id: null, total: { $sum: "price" } } }

B. { $group: { _id: null, total: { $sum: "$price" } } }

C. { $group: { _id: null, total: { $sum: price } } }

D. { $group: { _id: null, total: { sum: "$price" } } }

Solution

1. Step 1: Identify correct field reference syntax

   In MongoDB aggregation, field names must be prefixed with "$" inside expressions, so "$price" is correct.

2. Step 2: Check accumulator syntax

   The accumulator must be $sum, not sum, and the value must be a string with "$" prefix.

3. Final Answer:

   { $group: { _id: null, total: { $sum: "$price" } } } -> Option B

4. Quick Check:

   Use $sum with "$field" syntax [OK]

Hint: Use "$field" inside $sum in $group [OK]

Common Mistakes:

• Omitting the $ before field name
• Using sum instead of $sum
• Not quoting the field name

3. Given the collection documents:

{ "category": "A", "value": 10 }

{ "category": "A", "value": 20 }

{ "category": "B", "value": 5 }

What will be the output of this aggregation?

[{ $group: { _id: "$category", totalValue: { $sum: "$value" } } }]

medium

A. [{ "_id": "A", "totalValue": 30 }, { "_id": "B", "totalValue": 5 }]

B. [{ "_id": "A", "totalValue": 10 }, { "_id": "B", "totalValue": 5 }]

C. [{ "_id": "A", "totalValue": 20 }, { "_id": "B", "totalValue": 5 }]

D. [{ "_id": null, "totalValue": 35 }]

Solution

1. Step 1: Group documents by category

   Documents with category "A" are grouped together, and category "B" separately.

2. Step 2: Sum the "value" fields per group

   For category "A": 10 + 20 = 30; for category "B": 5.

3. Final Answer:

   [{ "_id": "A", "totalValue": 30 }, { "_id": "B", "totalValue": 5 }] -> Option A

4. Quick Check:

   Sum values per category [OK]

Hint: Sum values grouped by _id field [OK]

Common Mistakes:

• Summing without grouping
• Using null _id causing single group
• Adding only one document's value

4. Consider this aggregation pipeline:

[{ $group: { _id: "$type", total: { $sum: value } } }]

What is the error in this pipeline?

medium

A. The _id field cannot be a string

B. Missing $group stage

C. Cannot use $sum inside $group

D. Field name 'value' should be quoted and prefixed with $ inside $sum

Solution

1. Step 1: Check field reference inside $sum

   The field 'value' must be referenced as "$value" (string with $ prefix) inside $sum.

2. Step 2: Confirm correct $group usage

   The $group stage is present and _id is correctly set to "$type".

3. Final Answer:

   Field name 'value' should be quoted and prefixed with $ inside $sum -> Option D

4. Quick Check:

   Use "$field" inside $sum [OK]

Hint: Always quote and prefix field with $ inside $sum [OK]

Common Mistakes:

• Using unquoted field names inside $sum
• Thinking $sum can't be used in $group
• Misunderstanding _id usage

5. You have a collection of sales documents with fields store and amount. You want to find the total sales per store but only for stores with total sales above 1000. Which aggregation pipeline correctly uses $sum and filters the results?

hard

A. [{ $match: { amount: { $gt: 1000 } } }, { $group: { _id: "$store", totalSales: { $sum: "$amount" } } }]

B. [{ $group: { _id: "$store", totalSales: { $sum: "$amount" } } }, { $match: { amount: { $gt: 1000 } } }]

C. [{ $group: { _id: "$store", totalSales: { $sum: "$amount" } } }, { $match: { totalSales: { $gt: 1000 } } }]

D. [{ $group: { _id: "$store", totalSales: { $sum: "$amount" } } }, { $match: { _id: { $gt: 1000 } } }]

Solution

1. Step 1: Group sales by store and sum amounts

   The first stage groups documents by "store" and sums the "amount" field into "totalSales".

2. Step 2: Filter groups with totalSales greater than 1000

   The second stage uses $match on the aggregated field "totalSales" to keep only stores with totals above 1000.

3. Final Answer:

   [{ $group: { _id: "$store", totalSales: { $sum: "$amount" } } }, { $match: { totalSales: { $gt: 1000 } } }] -> Option C

4. Quick Check:

   Group then filter on sum field [OK]

Hint: Group first, then filter on summed field [OK]

Common Mistakes:

• Filtering before grouping
• Filtering on original amount instead of totalSales
• Filtering on wrong field name