Array update with $[] all positional in MongoDB - Mini Project: Build & Apply

Update All Elements in a MongoDB Array Using $[] Positional Operator

📖 Scenario: You manage a MongoDB collection that stores information about books in a library. Each book document contains an array of reviews, where each review has a rating and a comment. You want to update all reviews to increase their rating by 1 point.

🎯 Goal: Build a MongoDB update query that uses the $[] all positional operator to increment the rating field of every review in the reviews array for a specific book.

📋 What You'll Learn

Create a collection named books with one document containing a title and a reviews array with exactly two review objects.

Define a filter variable filter to select the book with the exact title 'Learn MongoDB'.

Write an update query using $inc and the $[] all positional operator to increment the rating of all reviews by 1.

Complete the update command by combining the filter and update variables in a db.books.updateOne() call.

💡 Why This Matters

🌍 Real World

Updating all elements in an array field is common when you want to apply the same change to every item, such as increasing ratings, marking all tasks as done, or resetting flags.

💼 Career

Knowing how to use the $[] positional operator is important for database developers and administrators working with MongoDB to efficiently update array data without multiple queries.

1

Create the books collection with one book document

Create a variable called books and assign it an array with one object. The object must have a title field set to 'Learn MongoDB' and a reviews array containing exactly two objects: the first with rating 3 and comment 'Good intro', the second with rating 4 and comment 'Very helpful'.

MongoDB

# Your code here

Hint

Remember to create a list with one dictionary. The dictionary should have the exact keys and values as specified.

2

Define the filter to select the book by title

Create a variable called filter and assign it a dictionary that matches documents where the title is exactly 'Learn MongoDB'.

MongoDB

books = [{
    'title': 'Learn MongoDB',
    'reviews': [
        {'rating': 3, 'comment': 'Good intro'},
        {'rating': 4, 'comment': 'Very helpful'}
    ]
}]

# Your code here

Hint

Use a dictionary with the key title and the exact string value.

3

Write the update query using $[] to increment all ratings

Create a variable called update and assign it a dictionary that uses $inc with the key 'reviews.$[].rating' set to 1 to increment all rating fields inside the reviews array.

MongoDB

books = [{
    'title': 'Learn MongoDB',
    'reviews': [
        {'rating': 3, 'comment': 'Good intro'},
        {'rating': 4, 'comment': 'Very helpful'}
    ]
}]

filter = {'title': 'Learn MongoDB'}

# Your code here

Hint

Use the $inc operator with the key reviews.$[].rating to update all array elements.

4

Complete the update command with db.books.updateOne()

Write a line that calls db.books.updateOne() with the filter and update variables as arguments to update the document.

MongoDB

books = [{
    'title': 'Learn MongoDB',
    'reviews': [
        {'rating': 3, 'comment': 'Good intro'},
        {'rating': 4, 'comment': 'Very helpful'}
    ]
}]

filter = {'title': 'Learn MongoDB'}

update = {'$inc': {'reviews.$[].rating': 1}}

# Your code here

Hint

Use db.books.updateOne(filter, update) to apply the update.

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