$inc operator for incrementing in MongoDB - Step-by-Step Execution

Concept Flow - $inc operator for incrementing

Start with document

↓

Apply $inc operator

↓

Find field to increment

↓

Add increment value to field

↓

Update document with new value

↓

Return updated document

The $inc operator finds a field in a document and adds a number to it, updating the document with the new value.

Execution Sample

MongoDB

db.inventory.updateOne(
  { item: "apple" },
  { $inc: { quantity: 5 } }
)

This command finds the document where item is 'apple' and adds 5 to its quantity field.

Execution Table

Step	Document Before	Operation	Field	Increment Value	Document After
1	{ item: "apple", quantity: 10 }	Find document	N/A	N/A	{ item: "apple", quantity: 10 }
2	{ item: "apple", quantity: 10 }	Apply $inc	quantity	5	{ item: "apple", quantity: 15 }
3	{ item: "apple", quantity: 15 }	Update document	quantity	N/A	{ item: "apple", quantity: 15 }

💡 Update complete, quantity incremented from 10 to 15

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
quantity	10	10	15	15

Key Moments - 2 Insights

What happens if the field to increment does not exist in the document?

If the field does not exist, $inc creates the field and sets it to the increment value. This is shown in the execution_table where the field is found or created before incrementing.

Can $inc be used to decrement a value?

Yes, by using a negative number as the increment value, $inc decreases the field's value. The execution_table would show the increment value as negative.

Visual Quiz - 3 Questions

Look at the execution_table, what is the quantity after step 2?

A 15

B 10

C 5

D 20

💡 Hint

Check the 'Document After' column in row for step 2 in execution_table.

At which step is the $inc operator applied?

A Step 1

B Step 3

C Step 2

D No step applies $inc

💡 Hint

Look at the 'Operation' column in execution_table to find when $inc is applied.

If the increment value was -3 instead of 5, what would be the quantity after step 2?

A 13

B 7

C 15

D 3

💡 Hint

Subtract 3 from the starting quantity 10 as shown in variable_tracker.

Concept Snapshot

$inc operator:
- Adds a number to a field's value in a document.
- If field missing, creates it with increment value.
- Use positive to increase, negative to decrease.
- Syntax: { $inc: { field: value } }
- Updates document atomically.

Full Transcript

The $inc operator in MongoDB updates a document by adding a specified number to a field's current value. If the field does not exist, it creates the field with that number. For example, if a document has quantity 10 and we apply $inc with 5, the quantity becomes 15. This operation is atomic and efficient for counters or numeric updates. Negative values decrease the field. The execution steps show finding the document, applying the increment, and updating the document with the new value.