$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.