$inc operator for incrementing in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $inc operator do in MongoDB?

The $inc operator increases or decreases the value of a field by a specified amount. It is used to update numeric values without replacing the whole document.

beginner

How do you increment a field named score by 5 using $inc?

Use { $inc: { score: 5 } } in the update query to add 5 to the current value of score.

beginner

Can $inc be used to decrement a value? How?

Yes, by using a negative number. For example, { $inc: { score: -3 } } decreases the score by 3.

intermediate

What happens if the field you want to increment does not exist?

MongoDB creates the field and sets it to the increment value. For example, if score doesn't exist, { $inc: { score: 1 } } creates score with value 1.

intermediate

Is $inc atomic in MongoDB? Why is this important?

Yes, $inc is atomic, meaning it safely increments the value even if multiple updates happen at the same time. This prevents conflicts and data loss.

What does { $inc: { count: 2 } } do in MongoDB?

A Adds 2 to the current value of <code>count</code>

B Sets <code>count</code> to 2

C Deletes the <code>count</code> field

D Multiplies <code>count</code> by 2

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

If score does not exist, what will { $inc: { score: 1 } } do?

A Set <code>score</code> to null

B Throw an error

C Ignore the update

D Create <code>score</code> with value 1

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

How do you decrease a field value by 4 using $inc?

A { $dec: { field: 4 } }

B { $inc: { field: 4 } }

C { $inc: { field: -4 } }

D { $dec: { field: -4 } }

Using a negative number with $inc decreases the field value.

Is $inc operation atomic in MongoDB?

A No, it can cause conflicts

B Yes, it prevents race conditions

C Only on replica sets

D Only on sharded clusters

$inc is atomic, so it safely updates values even with concurrent operations.

Which of the following is a valid use of $inc?

A { $inc: { age: 1 } }

B { $inc: { status: true } }

C { $inc: { active: 'yes' } }

D { $inc: { name: 1 } }

$inc works only on numeric fields like age.

Explain how the $inc operator works in MongoDB and give an example of incrementing and decrementing a field.

Think about adding or subtracting numbers from a field.

Why is atomicity important for the $inc operator in MongoDB?

Consider what happens if two people try to update the same number at once.

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