$inc operator for incrementing in MongoDB

Introduction

The $inc operator is used to add or subtract a number from a field's current value in a document. It helps update numbers easily without replacing the whole value.

When you want to increase a user's score after they complete a task.

When you need to decrease the stock quantity after a sale.

When you want to count how many times an event happens by adding 1 each time.

When you want to adjust a balance by adding or subtracting an amount.

When you want to update a numeric field without fetching the whole document first.

Syntax

MongoDB

db.collection.updateOne(
  { <filter> },
  { $inc: { <field1>: <amount1>, <field2>: <amount2>, ... } }
)

The $inc operator takes an object where keys are field names and values are numbers to add (positive) or subtract (negative).

If the field does not exist, $inc will create it and set it to the increment value.

Examples

Adds 5 to Alice's score field.

MongoDB

db.users.updateOne({ name: "Alice" }, { $inc: { score: 5 } })

Subtracts 3 from the stock of the Pen product.

MongoDB

db.products.updateOne({ item: "Pen" }, { $inc: { stock: -3 } })

Increments the pageViews count by 1.

MongoDB

db.counters.updateOne({ _id: "pageViews" }, { $inc: { count: 1 } })

Sample Program

This example first adds a document with 10 notebooks. Then it increases the quantity by 5 using $inc. Finally, it shows the updated document.

MongoDB

db.inventory.insertOne({ item: "notebook", qty: 10 })
db.inventory.updateOne({ item: "notebook" }, { $inc: { qty: 5 } })
db.inventory.find({ item: "notebook" }).toArray()

Important Notes

You can increment multiple fields at once by listing them inside the $inc object.

Using a negative number with $inc subtracts from the field.

If the field is not a number, $inc will cause an error.

Summary

$inc updates numeric fields by adding or subtracting values.

It creates the field if it does not exist.

Use it to change numbers without replacing the whole document.

Practice

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

easy

A. It creates a new collection.

B. It increments or decrements the value of a numeric field.

C. It deletes a field from the document.

D. It replaces the entire document with a new one.

Solution

1. Step 1: Understand the purpose of $inc

   The $inc operator is used to add or subtract a number from an existing numeric field in a document.

2. Step 2: Compare with other options

   Replacing documents, deleting fields, or creating collections are not functions of $inc.

3. Final Answer:

   It increments or decrements the value of a numeric field. -> Option B

4. Quick Check:

   $inc changes numbers by adding/subtracting [OK]

Hint: Remember: $inc changes numbers, not documents [OK]

Common Mistakes:

• Thinking $inc replaces the whole document
• Confusing $inc with delete or create operations
• Assuming $inc works on non-numeric fields

2. Which of the following is the correct syntax to increment the field score by 5 in MongoDB?

easy

A. { $inc: { score: '5' } }

B. { $inc: { score: '+5' } }

C. { $inc: { 'score': 'five' } }

D. { $inc: { score: 5 } }

Solution

1. Step 1: Check the value type for $inc

   The value to increment must be a number, not a string or word.

2. Step 2: Validate syntax correctness

   { $inc: { score: 5 } } uses a number 5 correctly. { $inc: { score: '5' } } uses a string '5', which is invalid. { $inc: { 'score': 'five' } } uses a word 'five', invalid. { $inc: { score: '+5' } } uses a string '+5', invalid.

3. Final Answer:

   { $inc: { score: 5 } } -> Option D

4. Quick Check:

   Use number values with $inc [OK]

Hint: Use numeric values without quotes for $inc [OK]

Common Mistakes:

• Using strings instead of numbers for increment values
• Adding plus sign (+) inside JSON value
• Using words instead of numeric literals

3. Given a document { _id: 1, count: 10 }, what will be the document after running db.collection.updateOne({ _id: 1 }, { $inc: { count: -3 } })?

medium

A. { _id: 1, count: -3 }

B. { _id: 1, count: 13 }

C. { _id: 1, count: 7 }

D. { _id: 1, count: 10 }

Solution

1. Step 1: Understand the initial document

   The document has count equal to 10.

2. Step 2: Apply the $inc operation with -3

   Subtract 3 from 10: 10 - 3 = 7.

3. Final Answer:

   { _id: 1, count: 7 } -> Option C

4. Quick Check:

   10 + (-3) = 7 [OK]

Hint: Subtract by using negative numbers with $inc [OK]

Common Mistakes:

• Adding instead of subtracting when using negative values
• Replacing the whole document instead of updating
• Assuming $inc only increments, not decrements

4. You run this update: db.collection.updateOne({ _id: 2 }, { $inc: { visits: 1 } }) but get an error. What is the most likely cause?

medium

A. The visits field is a string, not a number.

B. The _id field is missing in the query.

C. The $inc operator cannot increment by 1.

D. The collection does not exist.

Solution

1. Step 1: Understand $inc requirements

   $inc only works on numeric fields. If visits is a string, it causes an error.

2. Step 2: Check other options

   Missing _id would not cause this error if the document exists. $inc can increment by 1. Collection existence error would be different.

3. Final Answer:

   The visits field is a string, not a number. -> Option A

4. Quick Check:

   $inc needs numeric fields [OK]

Hint: Ensure field is numeric before using $inc [OK]

Common Mistakes:

• Trying to increment string fields
• Assuming $inc works on any data type
• Ignoring error messages about field types

5. You want to increment the likes field by 1 for all documents where likes does not exist yet. Which update command will correctly do this without errors?

hard

A. db.collection.updateMany({ likes: { $exists: false } }, { $inc: { likes: 1 } })

B. db.collection.updateMany({}, { $inc: { likes: 1 } })

C. db.collection.updateMany({ likes: null }, { $inc: { likes: 1 } })

D. db.collection.updateMany({}, { $set: { likes: 1 } })

Solution

1. Step 1: Identify documents without likes

   Use the query { likes: { $exists: false } } to find documents missing the likes field.

2. Step 2: Use $inc to add 1 to likes

   $inc will create the field with value 1 if it does not exist, so this safely increments missing fields.

3. Step 3: Check other options

   db.collection.updateMany({}, { $inc: { likes: 1 } }) increments all documents, including those with likes. db.collection.updateMany({ likes: null }, { $inc: { likes: 1 } }) matches documents where likes is null, not missing. db.collection.updateMany({}, { $set: { likes: 1 } }) sets likes to 1, not increments.

4. Final Answer:

   db.collection.updateMany({ likes: { $exists: false } }, { $inc: { likes: 1 } }) -> Option A

5. Quick Check:

   Increment missing fields with $exists: false filter [OK]

Hint: Filter missing fields with $exists: false before $inc [OK]

Common Mistakes:

• Incrementing all documents without filtering
• Using $set instead of $inc
• Filtering with null instead of $exists: false