$addFields for computed fields in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this $addFields stage?

Given a collection with documents like {"name": "Alice", "scores": [10, 20, 30]}, what will be the result of this aggregation pipeline stage?

{ "$addFields": { "totalScore": { "$sum": "$scores" } }}

MongoDB

db.collection.aggregate([{ "$addFields": { "totalScore": { "$sum": "$scores" } }}])

A [{"name": "Alice", "scores": [10, 20, 30], "totalScore": 60}]

B [{"name": "Alice", "scores": [10, 20, 30], "totalScore": [10, 20, 30]}]

C [{"name": "Alice", "scores": [10, 20, 30], "totalScore": 3}]

D [{"name": "Alice", "scores": [10, 20, 30], "totalScore": null}]

💡 Hint

Think about what $sum does when applied to an array field.

The $sum operator sums all numbers in the array 'scores', so 10 + 20 + 30 = 60. The $addFields stage adds this as a new field 'totalScore'.

❓ query_result

intermediate

What does this $addFields expression compute?

Consider documents with fields {"price": 100, "tax": 0.15}. What will this $addFields stage add?

{ "$addFields": { "finalPrice": { "$multiply": ["$price", { "$add": [1, "$tax"] }] } }}

MongoDB

db.collection.aggregate([{ "$addFields": { "finalPrice": { "$multiply": ["$price", { "$add": [1, "$tax"] }] } }}])

A [{"price": 100, "tax": 0.15, "finalPrice": 100.15}]

B [{"price": 100, "tax": 0.15, "finalPrice": 15}]

C [{"price": 100, "tax": 0.15, "finalPrice": 1.15}]

D [{"price": 100, "tax": 0.15, "finalPrice": 115}]

💡 Hint

Calculate 100 * (1 + 0.15).

The expression calculates finalPrice as price multiplied by (1 + tax), so 100 * 1.15 = 115.

📝 Syntax

advanced

Which $addFields syntax is valid to add a field 'discounted' as price minus discount?

Choose the correct $addFields stage to compute 'discounted' = price - discount.

A { "$addFields": { "discounted": { "$subtract": ["$price", "$discount"] } } }

B { "$addFields": { "discounted": "$price" - "$discount" } }

C { "$addFields": { "discounted": { "$minus": ["$price", "$discount"] } } }

D { "$addFields": { "discounted": { "$subtract": "$price" - "$discount" } } }

💡 Hint

Use the correct MongoDB operator for subtraction with array syntax.

The $subtract operator requires an array of two expressions to subtract. Option A uses correct syntax.

❓ optimization

advanced

How to optimize adding multiple computed fields in one $addFields stage?

You want to add 'total' as sum of 'a' and 'b', and 'average' as their average. Which $addFields stage is best?

A { "$addFields": { "total": { "$add": ["$a", "$b"] } } }, { "$addFields": { "average": { "$divide": [{ "$add": ["$a", "$b"] }, 2] } } }

B [{ "$addFields": { "total": { "$add": ["$a", "$b"] } } }, { "$addFields": { "average": { "$divide": [{ "$add": ["$a", "$b"] }, 2] } } }]

C { "$addFields": { "total": { "$add": ["$a", "$b"] }, "average": { "$divide": [{ "$add": ["$a", "$b"] }, 2] } } }

D { "$addFields": { "average": { "$divide": [{ "$add": ["$a", "$b"] }, 2] } }, { "$addFields": { "total": { "$add": ["$a", "$b"] } } } }

💡 Hint

Combine all computed fields in a single $addFields stage for efficiency.

Option C adds both fields in one stage, reducing pipeline stages and improving performance.

🧠 Conceptual

expert

What happens if $addFields uses a field name that already exists?

If a document has a field 'status' and you use $addFields to add 'status' with a new value, what is the result?

A MongoDB throws an error because field names must be unique.

B The existing 'status' field is replaced with the new value from $addFields.

C The new 'status' field is added alongside the existing one as an array.

D The existing 'status' field is kept and the new one is ignored.

💡 Hint

Think about how $addFields modifies documents in aggregation.

$addFields overwrites existing fields if the same name is used, replacing old values with new ones.