Why Array update with $[] all positional in MongoDB? - Purpose & Use Cases

The Big Idea

What if you could update every item in a list with just one simple command?

The Scenario

Imagine you have a list of tasks for a project stored on paper. You want to mark all tasks as completed, but you have to cross out each one by hand, one after another.

The Problem

Doing this manually is slow and easy to miss some tasks. If the list is long, it becomes frustrating and error-prone. You might forget to update some tasks or make mistakes crossing them out.

The Solution

Using MongoDB's $[] all positional operator, you can update every item in an array at once with a single command. This saves time and ensures no item is missed.

Before vs After

✗ Before

db.projects.updateOne({_id:1}, {$set: {'tasks.0.completed': true}})
db.projects.updateOne({_id:1}, {$set: {'tasks.1.completed': true}})
... (repeat for each task)

✓ After

db.projects.updateOne({_id:1}, {$set: {'tasks.$[].completed': true}})

What It Enables

You can quickly and safely update all elements in an array without writing repetitive code or risking missing any item.

Real Life Example

For a shopping app, if a sale applies to all items in a user's cart, you can instantly mark every item as discounted using $[] instead of updating each product one by one.

Key Takeaways

Manual updates of array items are slow and error-prone.

$[] lets you update all array elements in one step.

This makes bulk updates simple, fast, and reliable.

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