Array update with $[] all positional in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to update all elements in the array field 'scores' by setting them to 100.

MongoDB

db.students.updateMany({}, { $set: { "scores[1]": 100 } })

Drag options to blanks, or click blank then click option'

A.$[]

B.$

C.[0]

D.$[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Using .$ instead of .$[] updates only the first element.

Using array index like [0] updates only one element.

The $[] operator updates all elements in the array.

2fill in blank

medium

Complete the code to increment all elements in the 'grades' array by 5.

MongoDB

db.courses.updateMany({}, { $inc: { "grades[1]": 5 } })

Drag options to blanks, or click blank then click option'

A.$[1]

B.$[]

C.$

D.[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Using .$ increments only the first element.

Using an index like [0] increments only one element.

The $[] operator targets all elements in the array for the increment.

3fill in blank

hard

Fix the error in the code to set all elements in 'ratings' array to 5.

MongoDB

db.products.updateMany({}, { $set: { "ratings[1]": 5 } })

Drag options to blanks, or click blank then click option'

A.[0]

B.$[elem]

C.$[]

D.$

Attempts:

3 left

💡 Hint

Common Mistakes

Using .$ updates only the first element, not all.

Using array index updates only one element.

Using $[] updates all elements; .$ updates only the first.

4fill in blank

hard

Fill both blanks to multiply all elements in 'values' array by 2.

MongoDB

db.data.updateMany({}, { $mul: { "values[1]": [2] } })

Drag options to blanks, or click blank then click option'

A.$[]

B2

C3

D.$

Attempts:

3 left

💡 Hint

Common Mistakes

Using .$ updates only the first element.

Multiplying by 3 instead of 2 changes the result.

The $[] operator targets all array elements, and $mul multiplies by the given number.

5fill in blank

hard

Fill both blanks to add 10 to all elements in 'scores' array and set 'updated' to true.

MongoDB

db.records.updateMany({}, { $inc: { "scores[1]": [2] }, $set: { "updated": true } })

Drag options to blanks, or click blank then click option'

A.$[]

B10

D.$

Attempts:

3 left

💡 Hint

Common Mistakes

Using .$ instead of .$[] updates only one element.

Adding wrong number instead of 10.

Using array operator on 'updated' field which is not an array.

$[] updates all array elements, $inc adds 10, and $set sets the 'updated' field (no array operator needed).

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