Array update with $[] all positional in MongoDB - Step-by-Step Execution

Concept Flow - Array update with $[] all positional

Start with document containing array

↓

Use update with $[

↓

MongoDB targets ALL elements in array

↓

Apply update to each element

↓

Save updated document

↓

End with all array elements updated

This flow shows how MongoDB uses the $[] operator to update every element in an array inside a document.

Execution Sample

MongoDB

db.inventory.updateMany(
  {},
  { $set: { "sizes.$[]": "medium" } }
)

This query updates all elements in the 'sizes' array to 'medium' for every document in the 'inventory' collection.

Execution Table

Step	Document Before	Update Operation	Array Targeted	Array Elements Before	Array Elements After	Document After
1	{"_id":1, "sizes": ["small", "large", "x-large"]}	{$set: {"sizes.$[]": "medium"}}	sizes	["small", "large", "x-large"]	["medium", "medium", "medium"]	{"_id":1, "sizes": ["medium", "medium", "medium"]}
2	{"_id":2, "sizes": ["small", "medium"]}	{$set: {"sizes.$[]": "medium"}}	sizes	["small", "medium"]	["medium", "medium"]	{"_id":2, "sizes": ["medium", "medium"]}
3	No more documents	No update	N/A	N/A	N/A	No change

💡 All documents processed; all array elements updated to 'medium'.

Variable Tracker

Variable	Start	After Doc 1	After Doc 2	Final
sizes array elements	["small", "large", "x-large"]	["medium", "medium", "medium"]	["medium", "medium"]	All arrays updated to 'medium'

Key Moments - 2 Insights

Why does $[] update all elements instead of just one?

Because $[] is the all positional operator in MongoDB, it targets every element in the array, as shown in execution_table rows 1 and 2 where all elements change.

What happens if the array is empty?

If the array is empty, $[] has no elements to update, so the array remains unchanged. This is implied in the exit_note when no more documents or elements exist.

Visual Quiz - 3 Questions

Look at the execution_table row 1, what is the value of the 'sizes' array after the update?

A ["small", "large", "x-large"]

B ["medium", "large", "x-large"]

C ["medium", "medium", "medium"]

D ["small", "medium", "x-large"]

💡 Hint

Check the 'Array Elements After' column in row 1 of the execution_table.

At which step does the update operation stop?

A After step 1

B After step 3

C After step 2

D It never stops

💡 Hint

Look at the exit_note and the last row in execution_table indicating no more documents.

If the array 'sizes' had one element 'large' only, what would be the array after update?

A ["medium"]

B ["large"]

C []

D ["small"]

💡 Hint

Refer to how $[] updates all elements in the array as shown in variable_tracker and execution_table.

Concept Snapshot

MongoDB $[] operator updates all elements in an array.
Syntax: { $set: { "arrayField.$[]": value } }
Targets every element inside the array.
Useful for bulk updating array elements without specifying indexes.
If array is empty, no changes occur.
Works inside updateOne, updateMany commands.

Full Transcript

This visual execution trace shows how MongoDB's $[] all positional operator updates every element in an array field inside documents. Starting with documents containing arrays, the update command with $[] targets all elements in the specified array. Each element is set to the new value, and the document is saved with updated arrays. The execution table tracks each document before and after the update, showing the array elements changing from their original values to the new value. The variable tracker highlights how the array elements change step-by-step. Key moments clarify why $[] updates all elements and what happens if the array is empty. The visual quiz tests understanding by referencing specific steps and outcomes in the execution table and variable tracker. This concept is essential for bulk updating array elements efficiently in MongoDB.