$sum accumulator in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Calculate total sales per product

Given a collection sales with documents containing product and amount, what is the output of this aggregation pipeline?

MongoDB

db.sales.aggregate([
  { $group: { _id: "$product", totalSales: { $sum: "$amount" } } }
])

A [{ "_id": "apple", "totalSales": 150 }, { "_id": "banana", "totalSales": 200 }]

B [{ "_id": "apple", "totalSales": 350 }, { "_id": "banana", "totalSales": 200 }]

C [{ "_id": "apple", "totalSales": 150 }, { "_id": "banana", "totalSales": 100 }]

D [{ "_id": "apple", "totalSales": 0 }, { "_id": "banana", "totalSales": 0 }]

💡 Hint

Sum the amounts grouped by product name.

The $sum accumulator adds all the amounts for each product. The output shows total sales per product.

🧠 Conceptual

intermediate

Understanding $sum with constant values

What does the following aggregation pipeline output if applied to any collection?

MongoDB

db.collection.aggregate([
  { $group: { _id: null, count: { $sum: 1 } } }
])

A [{ "_id": null, "count": <number_of_documents> }]

B [{ "_id": null, "count": 0 }]

C [{ "_id": null, "count": null }]

D SyntaxError

💡 Hint

Think about what summing 1 for each document means.

Using $sum: 1 counts the number of documents because it adds 1 for each document.

📝 Syntax

advanced

Identify the syntax error in $sum usage

Which option contains a syntax error in using $sum inside a $group stage?

MongoDB

db.orders.aggregate([
  { $group: { _id: "$customer", total: { $sum: "price" } } }
])

A Using $sum: "$price" (with $ before field name) is correct syntax.

B Using $sum: ["$price"] is correct syntax.

C Using $sum: { $price: 1 } is correct syntax.

D Using $sum: "price" (without $) is correct syntax.

💡 Hint

Field names must be prefixed with $ inside $sum to refer to fields.

The $sum accumulator requires a field path prefixed with $ to sum field values. Using a string without $ causes an error.

❓ optimization

advanced

Optimizing $sum with $match before $group

Which pipeline is more efficient to calculate total sales for products with amount > 100?

A db.sales.aggregate([{ $group: { _id: "$product", total: { $sum: "$amount" } } }])

B db.sales.aggregate([{ $group: { _id: "$product", total: { $sum: "$amount" } } }, { $match: { total: { $gt: 100 } } }])

C db.sales.aggregate([{ $match: { amount: { $gt: 100 } } }, { $group: { _id: "$product", total: { $sum: "$amount" } } }])

D db.sales.aggregate([{ $group: { _id: "$product", total: { $sum: { $cond: [{ $gt: ["$amount", 100] }, "$amount", 0] } } } }])

💡 Hint

Filtering early reduces documents processed in grouping.

Applying $match before $group reduces the number of documents to group, improving performance.

🔧 Debug

expert

Why does this $sum aggregation return incorrect totals?

Given this pipeline, why might the total be incorrect?

MongoDB

db.orders.aggregate([
  { $group: { _id: "$customer", total: { $sum: "$items.price" } } }
])

A Because $sum requires numeric literals, not field paths.

B Because $sum cannot sum values inside arrays without $unwind first.

C Because $group cannot use nested fields in _id.

D Because $sum only works with top-level fields, not nested.

💡 Hint

Think about summing values inside arrays.

To sum values inside an array field, you must unwind the array first to flatten documents.