$inc operator for incrementing in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to increment the field 'score' by 1 in a MongoDB update.

MongoDB

db.collection.updateOne({ _id: 1 }, { [1]: { score: 1 } })

Drag options to blanks, or click blank then click option'

A$set

B$inc

C$push

D$addToSet

Attempts:

3 left

💡 Hint

Common Mistakes

Using $set instead of $inc will replace the value instead of incrementing.

Using $push or $addToSet are for arrays, not numbers.

The $inc operator increments the value of a field by the specified amount.

2fill in blank

medium

Complete the code to increment the 'views' field by 5 in a MongoDB update.

MongoDB

db.collection.updateOne({ _id: 2 }, { [1]: { views: 5 } })

Drag options to blanks, or click blank then click option'

A$inc

B$min

C$mul

D$set

Attempts:

3 left

💡 Hint

Common Mistakes

Using $set will overwrite the value instead of incrementing.

Using $mul multiplies the value, not increments.

$inc adds the specified number to the current value of the field.

3fill in blank

hard

Fix the error in the update code to increment 'likes' by 3.

MongoDB

db.collection.updateOne({ _id: 3 }, { $inc: { likes: [1] } })

Drag options to blanks, or click blank then click option'

A'3'

B-3

C3

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Putting the number in quotes makes it a string, which is invalid.

Using true is not a number and causes an error.

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

4fill in blank

hard

Fill both blanks to increment 'count' by 2 and 'score' by 10 in one update.

MongoDB

db.collection.updateOne({ _id: 4 }, { [1]: { count: 2, [2]: 10 } })

Drag options to blanks, or click blank then click option'

A$inc

B$set

Cscore

D$mul

Attempts:

3 left

💡 Hint

Common Mistakes

Using $set will overwrite values instead of incrementing.

Using $mul multiplies values, not increments.

Use $inc to increment fields. The second blank is the field name 'score'.

5fill in blank

hard

Fill all three blanks to increment 'likes' by 1, 'views' by 5, and 'shares' by 2 in one update.

MongoDB

db.collection.updateOne({ _id: 5 }, { [1]: { likes: [2], views: [3], shares: 2 } })

Drag options to blanks, or click blank then click option'

A$set

B$inc

C1

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using $set will overwrite values instead of incrementing.

Putting numbers in quotes makes them strings, which is invalid.

Use $inc to increment fields. The values 1 and 5 are the amounts to add to 'likes' and 'views'.

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