$mul operator for multiplication in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this update operation?

Given a MongoDB collection products with a document:

{ "_id": 1, "price": 100, "quantity": 5 }

What will be the document after running this update?

db.products.updateOne({ _id: 1 }, { $mul: { price: 1.1, quantity: 2 } })

A { "_id": 1, "price": 110, "quantity": 7 }

B { "_id": 1, "price": 110, "quantity": 10 }

C { "_id": 1, "price": 101, "quantity": 7 }

D { "_id": 1, "price": 100, "quantity": 10 }

💡 Hint

The $mul operator multiplies the current field value by the given number.

The price 100 is multiplied by 1.1 resulting in 110. The quantity 5 is multiplied by 2 resulting in 10.

🧠 Conceptual

intermediate

Which statement about $mul is true?

Choose the correct statement about the MongoDB $mul operator.

A $mul adds the multiplier value to the current field value.

B $mul can multiply any field type including strings and arrays.

C $mul replaces the field value with the multiplier instead of multiplying.

D $mul can only multiply numeric fields and will error if used on non-numeric fields.

💡 Hint

Think about what happens if you try to multiply a string by a number.

$mul only works on numeric fields. Using it on non-numeric fields causes an error.

📝 Syntax

advanced

Which update command correctly multiplies the field 'score' by 3?

Select the valid MongoDB update command that multiplies the 'score' field by 3 for documents with {_id: 5}.

A db.collection.updateOne({ _id: 5 }, { $mul: { score: 3 } })

B db.collection.updateOne({ _id: 5 }, { $mul: { score: null } })

C db.collection.updateOne({ _id: 5 }, { $mul: { score: [3] } })

D db.collection.updateOne({ _id: 5 }, { $mul: { score: "3" } })

💡 Hint

The multiplier must be a number, not a string or other type.

Option A uses a numeric multiplier 3, which is valid. Options A, C, and D use invalid types causing errors.

🔧 Debug

advanced

Why does this update fail?

Given a document:

{ "_id": 10, "count": "5" }

Why does this update fail?

db.collection.updateOne({ _id: 10 }, { $mul: { count: 2 } })

A Because 'count' is a string, $mul cannot multiply it and throws an error.

B Because the multiplier 2 is not a string.

C Because the document does not have a numeric _id.

D Because $mul requires the field to be missing to create it.

💡 Hint

Check the data type of the field being multiplied.

The 'count' field is a string "5", not a number. $mul only works on numeric fields, so it fails.

❓ optimization

expert

How to efficiently multiply multiple fields in many documents?

You want to multiply the fields 'price' and 'tax' by 1.05 in all documents of a large collection. Which approach is best for performance?

A Fetch all documents, multiply fields in application code, then update each document individually.

B Run two separate updateMany commands, one for 'price' and one for 'tax'.

C Run a single updateMany with $mul on both fields: db.collection.updateMany({}, { $mul: { price: 1.05, tax: 1.05 } })

D Use $inc operator to add 5% of the current value to each field.

💡 Hint

Consider how many database operations are needed and atomicity.

Option C performs one atomic update on all documents, minimizing operations and ensuring consistency.