$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.

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