$mul operator for multiplication in MongoDB - Step-by-Step Execution

Concept Flow - $mul operator for multiplication

Start Update

↓

Find Document

↓

Apply $mul

↓

Multiply Field by Value

↓

Save Updated Document

↓

End Update

The $mul operator multiplies a field's value by a specified number during an update operation.

Execution Sample

MongoDB

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

This multiplies the 'price' field of the product with _id 1 by 1.1 (increasing price by 10%).

Execution Table

Step	Action	Field 'price' Value Before	Multiplier	Field 'price' Value After	Notes
1	Find document with _id:1	100	N/A	100	Initial price is 100
2	Apply $mul operator	100	1.1	110	100 * 1.1 = 110
3	Save updated document	N/A	N/A	110	Document updated with new price
4	End update	110	N/A	110	Update operation complete

💡 Update ends after multiplying 'price' by 1.1 and saving the document.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
price	100	100	110	110	110

Key Moments - 2 Insights

Why does the 'price' field change from 100 to 110?

Because the $mul operator multiplies the current 'price' value (100) by 1.1, resulting in 110 as shown in execution_table step 2.

What happens if the field does not exist in the document?

The $mul operator will create the field and set it to 0 multiplied by the value, which results in 0. This is not shown in the current example but is important to know.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'price' after step 2?

A 110

B 100

C 1.1

D 0

💡 Hint

Check the 'Field price Value After' column in step 2 of the execution_table.

At which step is the document saved with the updated price?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for the action 'Save updated document' in the execution_table.

If the multiplier was 2 instead of 1.1, what would be the 'price' after step 2?

A 110

B 200

C 100

D 50

💡 Hint

Multiply the initial price 100 by 2 as per the $mul operator logic.

Concept Snapshot

$mul operator syntax:
{ $mul: { <field>: <number> } }
Multiplies the field's current value by the number.
If field missing, treated as 0.
Used in update operations to scale numeric fields.

Full Transcript

The $mul operator in MongoDB updates multiplies a numeric field by a specified value. The process starts by finding the document to update. Then, $mul multiplies the current field value by the given multiplier. The updated value replaces the old one, and the document is saved. For example, multiplying a price of 100 by 1.1 results in 110. If the field does not exist, it is treated as zero. This operator is useful for scaling prices, quantities, or other numeric data in documents.

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