$inc operator for incrementing in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Increment a field using $inc

Given a MongoDB document { _id: 1, score: 10 }, what will be the document after applying { $inc: { score: 5 } } update?

MongoDB

db.collection.updateOne({ _id: 1 }, { $inc: { score: 5 } })
db.collection.findOne({ _id: 1 })

A { _id: 1, score: 5 }

B { _id: 1, score: 15 }

C { _id: 1, score: 10 }

D Error: $inc requires a numeric value

💡 Hint

Think about what adding 5 to 10 results in.

The $inc operator adds the specified value to the existing field value. Here, 10 + 5 = 15.

📝 Syntax

intermediate

Identify the correct $inc syntax

Which of the following update commands correctly increments the count field by 3 in MongoDB?

A db.collection.updateOne({}, { $inc: count: 3 })

B db.collection.updateOne({}, { inc: { count: 3 } })

C db.collection.updateOne({}, { $inc: { count: 3 } })

D db.collection.updateOne({}, { $inc: { count += 3 } })

💡 Hint

Remember the $inc operator must be inside an object with the field name as key.

Option C uses the correct MongoDB update syntax with $inc operator and a proper object for the field increment.

❓ optimization

advanced

Efficiently increment multiple fields

You want to increment likes by 2 and shares by 1 in a single update. Which update command is the most efficient?

A db.collection.updateOne({ _id: 1 }, { $inc: { likes: 2 } }); db.collection.updateOne({ _id: 1 }, { $inc: { shares: 1 } })

B db.collection.updateOne({ _id: 1 }, { $inc: { likes: 1, shares: 2 } })

C db.collection.updateOne({ _id: 1 }, { $set: { likes: likes + 2, shares: shares + 1 } })

D db.collection.updateOne({ _id: 1 }, { $inc: { likes: 2, shares: 1 } })

💡 Hint

Try to do both increments in one command.

Option D increments both fields in a single atomic update, which is efficient and safe.

🧠 Conceptual

advanced

Behavior of $inc on non-existing fields

What happens when you use { $inc: { views: 1 } } on a document that does not have the views field?

A The <code>views</code> field is created with value 1

B The <code>views</code> field is created with value 0

C The document remains unchanged

D The update fails with an error

💡 Hint

Think about how $inc treats missing fields.

MongoDB creates the field and sets it to the increment value if it does not exist.

🔧 Debug

expert

Diagnose the error in $inc usage

A developer runs this update: db.collection.updateOne({ _id: 1 }, { $inc: { score: 'five' } }). What error will MongoDB raise?

A Error: The $inc value must be a number

B No error, score becomes 'five'

C Error: Missing $set operator

D Error: Invalid query selector

💡 Hint

Check the type of the value used with $inc.

The $inc operator requires a numeric value; using a string causes an error.

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