$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.

Practice

1. What does the $mul operator do in MongoDB?

easy

A. It multiplies the value of a field by a specified number.

B. It adds a specified number to the value of a field.

C. It divides the value of a field by a specified number.

D. It subtracts a specified number from the value of a field.

Solution

1. Step 1: Understand the purpose of $mul

   The $mul operator is used to multiply the value of a numeric field by a given number in an update operation.

2. Step 2: Compare with other arithmetic operators

   Unlike $inc which adds, or $div which divides, $mul specifically multiplies the field value.

3. Final Answer:

   It multiplies the value of a field by a specified number. -> Option A

4. Quick Check:

   $mul = multiply [OK]

Hint: Remember: $mul means multiply field by number [OK]

Common Mistakes:

• Confusing $mul with $inc (addition)
• Thinking $mul divides instead of multiplies
• Assuming $mul works on non-numeric fields

2. Which of the following is the correct syntax to multiply the field price by 3 in a MongoDB update?

easy

A. { $mul: { price: price * 3 } }

B. { $mul: { 3: price } }

C. { $mul: { price: "3" } }

D. { $mul: { price: 3 } }

Solution

1. Step 1: Identify correct operator usage

   The $mul operator requires a field name as key and a numeric multiplier as value.

2. Step 2: Check each option's structure

   { $mul: { price: 3 } } correctly uses { $mul: { price: 3 } }. { $mul: { 3: price } } reverses key and value. { $mul: { price: "3" } } uses a string instead of number. { $mul: { price: price * 3 } } uses an invalid expression.

3. Final Answer:

   { $mul: { price: 3 } } -> Option D

4. Quick Check:

   Correct syntax = { $mul: { price: 3 } } [OK]

Hint: Use field name as key and number as value inside $mul [OK]

Common Mistakes:

• Using string instead of number for multiplier
• Swapping key and value inside $mul
• Trying to use expressions inside $mul

3. Given a document { _id: 1, quantity: 5, price: 10 }, what will be the document after running db.collection.updateOne({ _id: 1 }, { $mul: { quantity: 2, price: 3 } })?

medium

A. { _id: 1, quantity: 2, price: 3 }

B. { _id: 1, quantity: 10, price: 30 }

C. { _id: 1, quantity: 5, price: 10 }

D. { _id: 1, quantity: 7, price: 13 }

Solution

1. Step 1: Understand the update operation

   The $mul operator multiplies each specified field by the given number. Here, quantity is multiplied by 2 and price by 3.

2. Step 2: Calculate new field values

   Original quantity 5 * 2 = 10, original price 10 * 3 = 30.

3. Final Answer:

   { _id: 1, quantity: 10, price: 30 } -> Option B

4. Quick Check:

   5*2=10 and 10*3=30 [OK]

Hint: Multiply each field value by given number in $mul [OK]

Common Mistakes:

• Adding instead of multiplying
• Not updating all specified fields
• Confusing field names or values

4. You run this update: db.products.updateOne({ _id: 2 }, { $mul: { stock: '2' } }). What is the issue?

medium

A. The update query is missing $set operator.

B. The field name 'stock' is invalid.

C. The multiplier '2' is a string, should be a number.

D. The filter {_id: 2} is incorrect.

Solution

1. Step 1: Check the multiplier type

   The $mul operator requires the multiplier to be a number, not a string.

2. Step 2: Identify the error in the update

   Here, '2' is a string (in quotes), which causes a type error in MongoDB.

3. Final Answer:

   The multiplier '2' is a string, should be a number. -> Option C

4. Quick Check:

   Multiplier must be numeric, not string [OK]

Hint: Multiplier must be a number, not a string [OK]

Common Mistakes:

• Using quotes around numbers in $mul
• Assuming $set is needed with $mul
• Blaming filter instead of multiplier type

5. You want to double the score field only if it is greater than 10 in documents. Which update query correctly uses $mul with a filter?

hard

A. db.collection.updateMany({ score: { $gt: 10 } }, { $mul: { score: 2 } })

B. db.collection.updateMany({}, { $mul: { score: { $gt: 10, $mul: 2 } } })

C. db.collection.updateMany({ $mul: { score: 2 } }, { score: { $gt: 10 } })

D. db.collection.updateMany({ score: 2 }, { $mul: { score: 10 } })

Solution

1. Step 1: Understand filtering in updateMany

   The first argument is the filter to select documents where score is greater than 10.

2. Step 2: Apply $mul only to filtered documents

   The second argument uses $mul to multiply score by 2 for those documents.

3. Final Answer:

   db.collection.updateMany({ score: { $gt: 10 } }, { $mul: { score: 2 } }) -> Option A

4. Quick Check:

   Filter then multiply = db.collection.updateMany({ score: { $gt: 10 } }, { $mul: { score: 2 } }) [OK]

Hint: Filter documents first, then apply $mul in update [OK]

Common Mistakes:

• Trying to use $mul inside filter
• Swapping filter and update arguments
• Using invalid syntax mixing filter and update