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.

Practice

1. What does the $[] operator do in a MongoDB update query?

easy

A. It updates every element in an array inside the matched documents.

B. It updates only the first element of an array.

C. It removes all elements from an array.

D. It adds a new element to the array.

Solution

1. Step 1: Understand the role of $[]

   The $[] operator is called the all positional operator and targets all elements in an array during an update.

2. Step 2: Compare with other array update operators

   Unlike $ which updates the first matched element, $[] updates every element in the array.

3. Final Answer:

   It updates every element in an array inside the matched documents. -> Option A

4. Quick Check:

   $[] updates all array elements [OK]

Hint: Remember $[] means update all items in the array [OK]

Common Mistakes:

• Confusing $[] with $ which updates only one element
• Thinking $[] adds or removes elements
• Assuming $[] works outside arrays

2. Which of the following is the correct syntax to increment every element in the scores array by 1 using $[] in MongoDB?

easy

A. db.collection.updateMany({}, { $inc: { 'scores.$[]': 1 } })

B. db.collection.updateMany({}, { $set: { 'scores.$[]': 1 } })

C. db.collection.updateMany({}, { $push: { 'scores.$[]': 1 } })

D. db.collection.updateMany({}, { $inc: { 'scores': 1 } })

Solution

1. Step 1: Identify the operator to increment array elements

   The $inc operator increments numeric values. Using 'scores.$[]' targets all elements in the array.

2. Step 2: Check syntax correctness

   db.collection.updateMany({}, { $inc: { 'scores.$[]': 1 } }) correctly uses $inc with $[] to increment each element by 1. Other options either use wrong operators or wrong field paths.

3. Final Answer:

   db.collection.updateMany({}, { $inc: { 'scores.$[]': 1 } }) -> Option A

4. Quick Check:

   Use $inc with 'array.$[]' to increment all elements [OK]

Hint: Use $inc with 'array.$[]' to update all array elements [OK]

Common Mistakes:

• Using $set instead of $inc for increment
• Trying to increment the whole array instead of elements
• Using $push which adds elements instead of updating

3. Given the document { _id: 1, scores: [10, 20, 30] }, what will be the scores array after running this update?

db.collection.updateOne({_id:1}, { $inc: { 'scores.$[]': 5 } })

medium

A. [10, 25, 30]

B. [10, 20, 30]

C. [5, 5, 5]

D. [15, 25, 35]

Solution

1. Step 1: Understand the update operation

   The update uses $inc with 'scores.$[]', which increments every element in the scores array by 5.

2. Step 2: Calculate the new array values

   Original array: [10, 20, 30]. After increment: [10+5, 20+5, 30+5] = [15, 25, 35].

3. Final Answer:

   [15, 25, 35] -> Option D

4. Quick Check:

   Increment all array elements by 5 = [15, 25, 35] [OK]

Hint: Add increment to each element using 'array.$[]' [OK]

Common Mistakes:

• Thinking only first element updates
• Replacing array with single value
• Not applying increment to all elements

4. You want to update all elements in the ratings array by multiplying each by 2. Which update query will cause an error?

medium

A. db.collection.updateMany({}, { $set: { 'ratings.$[]': 2 } })

B. db.collection.updateMany({}, { $inc: { 'ratings.$[]': 2 } })

C. db.collection.updateMany({}, { $push: { 'ratings.$[]': 2 } })

D. db.collection.updateMany({}, { $mul: { 'ratings.$[]': 2 } })

Solution

1. Step 1: Analyze each update operator with $[]

   $mul, $inc, and $set can work with $[] to update array elements. $push is for adding elements, not updating existing ones.

2. Step 2: Identify the invalid usage

   Using $push with $[] is invalid and causes an error because $push expects a field name without positional operators.

3. Final Answer:

   db.collection.updateMany({}, { $push: { 'ratings.$[]': 2 } }) -> Option C

4. Quick Check:

   $push cannot be used with $[] [OK]

Hint: Only $inc, $mul, $set work with $[], not $push [OK]

Common Mistakes:

• Using $push with $[] causing syntax error
• Confusing $inc and $mul usage
• Trying to multiply array directly without $[]

5. You have documents with a field items as an array of objects like { _id: 1, items: [{qty: 2}, {qty: 5}] }. You want to add 3 to the qty of every item. Which update query correctly does this?

hard

A. db.collection.updateMany({}, { $set: { items: { $map: { input: "$items", as: "item", in: { qty: { $add: ["$$item.qty", 3] } } } } } })

B. db.collection.updateMany({}, { $inc: { 'items.$[].qty': 3 } })

C. db.collection.updateMany({}, { $set: { 'items.$[].qty': { $add: ['items.$[].qty', 3] } } })

D. db.collection.updateMany({}, { $push: { 'items.$[].qty': 3 } })

Solution

1. Step 1: Understand the structure and goal

   Each document has an array items with objects containing qty. We want to add 3 to each qty.

2. Step 2: Check update options

   db.collection.updateMany({}, { $inc: { 'items.$[].qty': 3 } }) uses $inc with items.$[].qty, which correctly increments the qty field of every object in the array. db.collection.updateMany({}, { $set: { items: { $map: { input: "$items", as: "item", in: { qty: { $add: ["$$item.qty", 3] } } } } } }) attempts to use aggregation operators like $map in a simple update, which requires pipeline syntax (array of stages) and is invalid. db.collection.updateMany({}, { $set: { 'items.$[].qty': { $add: ['items.$[].qty', 3] } } }) is invalid syntax. db.collection.updateMany({}, { $push: { 'items.$[].qty': 3 } }) misuses $push.

3. Final Answer:

   db.collection.updateMany({}, { $inc: { 'items.$[].qty': 3 } }) -> Option B

4. Quick Check:

   Use $inc with 'array.$[].field' to update nested fields [OK]

Hint: Use $inc with 'array.$[].field' to update nested fields [OK]

Common Mistakes:

• Using $push to update fields inside objects
• Trying to use $set with $add inside update document
• Not using $[] to target all array elements