$set operator for setting fields in MongoDB - Step-by-Step Execution

Concept Flow - $set operator for setting fields

Start with existing document

↓

Apply $set operator

↓

Check each field in $set

↓

If field exists, update value

↓

If field does not exist, add field with value

↓

Save updated document

↓

End

The $set operator updates specified fields in a document. It changes existing fields or adds new ones if they don't exist.

Execution Sample

MongoDB

db.collection.updateOne(
  { _id: 1 },
  { $set: { name: "Alice", age: 30 } }
)

This updates the document with _id 1, setting 'name' to 'Alice' and 'age' to 30.

Execution Table

Step	Document Before	$set Fields	Document After	Action
1	{ _id: 1, name: "Bob", city: "NY" }	{ name: "Alice", age: 30 }	{ _id: 1, name: "Alice", city: "NY", age: 30 }	Updated 'name', added 'age'
2	{ _id: 2, city: "LA" }	{ name: "Alice", age: 30 }	{ _id: 2, city: "LA", name: "Alice", age: 30 }	Added 'name' and 'age'
3	{ _id: 3, name: "Charlie", age: 25 }	{ age: 30 }	{ _id: 3, name: "Charlie", age: 30 }	Updated 'age' only
4	N/A	N/A	N/A	No more documents to update, operation ends

💡 All target documents processed, $set applied to each.

Variable Tracker

Document _id	Before $set	After $set
1	{ name: "Bob", city: "NY" }	{ name: "Alice", city: "NY", age: 30 }
2	{ city: "LA" }	{ city: "LA", name: "Alice", age: 30 }
3	{ name: "Charlie", age: 25 }	{ name: "Charlie", age: 30 }

Key Moments - 2 Insights

Why does $set add new fields if they don't exist?

Because $set updates the document by changing existing fields or adding new ones if missing, as shown in execution_table rows 1 and 2.

What happens if $set updates a field that already exists?

The existing field's value is replaced with the new value, as seen in execution_table row 3 where 'age' changed from 25 to 30.

Visual Quiz - 3 Questions

Look at the execution_table, what is the 'age' value for document _id 1 after $set?

A 25

B null

C 30

D Not present

💡 Hint

Check the 'Document After' column for step 1 in execution_table.

At which step does $set add both 'name' and 'age' fields to a document that had neither?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Document Before' and 'Document After' columns in execution_table for step 2.

If we only $set { age: 35 } on document _id 3, what would be the 'age' after update?

A 25

B 35

C 30

D Field removed

💡 Hint

Refer to how $set updates existing fields in execution_table row 3.

Concept Snapshot

$set operator updates fields in a MongoDB document.
It replaces existing field values or adds new fields if missing.
Syntax: { $set: { field1: value1, field2: value2 } }
Used in update operations like updateOne or updateMany.
Does not remove fields, only sets or adds.

Full Transcript

The $set operator in MongoDB is used to update specific fields in documents. When you apply $set, it changes the value of existing fields or adds new fields if they don't exist. For example, if a document has a field 'name' with value 'Bob', using $set to set 'name' to 'Alice' will update it. If the document does not have an 'age' field, $set will add it with the specified value. This operation is commonly used in update commands like updateOne. The execution table shows step-by-step how documents change before and after applying $set. Key points include that $set never removes fields, only sets or adds them, and it works on each targeted document individually.