Array update with $[identifier] filtered in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to update the first matching element in the array.

MongoDB

db.collection.updateOne({ _id: 1 }, { $set: { 'items.$[[1]]': 10 } }, { arrayFilters: [{ '[1].type': 'fruit' }] })

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

Aitem

Belem

Cindex

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names in the update path and arrayFilters.

Using reserved keywords like 'value' as identifier.

The $[identifier] is a placeholder for the array element to update. Here, item is used as the identifier in both the update path and the array filter.

2fill in blank

medium

Complete the code to increment the quantity of all matching array elements.

MongoDB

db.collection.updateMany({}, { $inc: { 'products.$[[1]].quantity': 1 } }, { arrayFilters: [{ '[1].status': 'available' }] })

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

Aindex

Bitem

Celem

Dprod

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatching identifier names.

Using invalid identifiers like 'index' which is not an object.

The identifier prod is used to refer to each array element in products that matches the filter status: 'available'.

3fill in blank

hard

Fix the error in the array update by choosing the correct identifier.

MongoDB

db.collection.updateOne({ _id: 5 }, { $set: { 'orders.$[[1]].status': 'shipped' } }, { arrayFilters: [{ '[1].status': 'pending' }] })

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

Aitem

Bord

Corder

Dorders

Attempts:

3 left

💡 Hint

Common Mistakes

Using different identifiers in update path and arrayFilters.

Using plural names for single array elements.

The identifier ord must be used consistently in both the update path and arrayFilters.

4fill in blank

hard

Fill the blanks to update the price of items with quantity less than 5.

MongoDB

db.collection.updateMany({}, { $set: { 'inventory.$[[1]].price': 20 } }, { arrayFilters: [{ '[2].quantity': { $[3]: 5 } }] })

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

Ainv

Blt

Attempts:

3 left

💡 Hint

Common Mistakes

Using different identifiers in update path and arrayFilters.

Using wrong comparison operators like $gt instead of $lt.

The identifier inv is used in both the update path and arrayFilters. The filter checks for quantity less than 5 using $lt.

5fill in blank

hard

Fill all three blanks to increment the score of players with level greater than 10.

MongoDB

db.players.updateMany({}, { $inc: { 'stats.$[[1]].score': [2] } }, { arrayFilters: [{ '[3].level': { $gt: 10 } }] })

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

Astat

B5

Dplayer

Attempts:

3 left

💡 Hint

Common Mistakes

Using different identifiers in update path and arrayFilters.

Using non-numeric values for increment.

The identifier stat is used consistently in the update path and arrayFilters. The score is incremented by 5 for players with level greater than 10.

Practice

1. What does the $[identifier] syntax do in a MongoDB update operation?

easy

A. It updates only the array elements that match the filter specified in arrayFilters.

B. It deletes the entire array from the document.

C. It adds a new element to the array without any condition.

D. It replaces the whole document with a new one.

Solution

1. Step 1: Understand $[identifier] usage

   The $[identifier] syntax is used to target specific elements inside an array during an update.

2. Step 2: Role of arrayFilters

   The arrayFilters option specifies which array elements the $[identifier] refers to, so only those elements get updated.

3. Final Answer:

   It updates only the array elements that match the filter specified in arrayFilters. -> Option A

4. Quick Check:

   $[identifier] updates filtered array items [OK]

Hint: Remember: $[id] updates filtered array elements only [OK]

Common Mistakes:

• Thinking $[identifier] updates the whole array
• Confusing $[identifier] with $push or $set without filters
• Ignoring the need for arrayFilters option

2. Which of the following is the correct syntax to update array elements using $[elem] with arrayFilters in MongoDB?

easy

A. db.collection.updateOne({},{ $set: { 'array.$[elem]': value } }, { arrayFilters: [ { 'elem.field': condition } ] })

B. db.collection.updateOne({},{ $push: { 'array.$[elem]': value } }, { arrayFilters: [ { 'elem.field': condition } ] })

C. db.collection.updateOne({},{ $set: { 'array.$elem': value } }, { arrayFilters: [ { 'elem.field': condition } ] })

D. db.collection.updateOne({},{ $set: { 'array.$[elem]': value } })

Solution

1. Step 1: Identify correct update operator and syntax

   The $set operator is used to update existing fields. The positional filtered identifier must be inside square brackets like $[elem].

2. Step 2: Confirm usage of arrayFilters

   The arrayFilters option must be provided with a filter object matching the identifier used inside $[elem].

3. Final Answer:

   db.collection.updateOne({},{ $set: { 'array.$[elem]': value } }, { arrayFilters: [ { 'elem.field': condition } ] }) -> Option A

4. Quick Check:

   Use $set with $[elem] and arrayFilters [OK]

Hint: Use $set with $[identifier] and arrayFilters together [OK]

Common Mistakes:

• Using $push instead of $set for updates
• Missing square brackets around identifier
• Not providing arrayFilters option

3. Given the collection document:

{ _id: 1, scores: [ { type: 'quiz', score: 5 }, { type: 'exam', score: 8 }, { type: 'quiz', score: 7 } ] }

What will be the result of this update?

db.collection.updateOne({ _id: 1 }, { $set: { 'scores.$[elem].score': 10 } }, { arrayFilters: [ { 'elem.type': 'quiz' } ] })

What will the scores array look like after the update?

medium

A. [ { type: 'quiz', score: 5 }, { type: 'exam', score: 8 }, { type: 'quiz', score: 7 } ]

B. [ { type: 'quiz', score: 10 }, { type: 'exam', score: 10 }, { type: 'quiz', score: 10 } ]

C. [ { type: 'quiz', score: 10 }, { type: 'exam', score: 8 }, { type: 'quiz', score: 7 } ]

D. [ { type: 'quiz', score: 10 }, { type: 'exam', score: 8 }, { type: 'quiz', score: 10 } ]

Solution

1. Step 1: Identify which array elements match the filter

   The filter { 'elem.type': 'quiz' } matches the first and third elements in the scores array.

2. Step 2: Apply the update to matched elements only

   The score field of these matched elements is set to 10, while the second element remains unchanged.

3. Final Answer:

   [ { type: 'quiz', score: 10 }, { type: 'exam', score: 8 }, { type: 'quiz', score: 10 } ] -> Option D

4. Quick Check:

   Only 'quiz' type scores updated to 10 [OK]

Hint: Only elements matching arrayFilters get updated [OK]

Common Mistakes:

• Updating all elements ignoring arrayFilters
• Updating wrong elements by misreading filter
• Expecting update without arrayFilters to work

4. You run this update:

db.collection.updateOne({ _id: 1 }, { $set: { 'items.$[i].status': 'sold' } }, { arrayFilters: [ { 'item.status': 'available' } ] })

But no documents are updated. What is the most likely cause?

medium

A. The $set operator cannot be used with $[i].

B. The query filter {_id: 1} is incorrect.

C. The identifier in arrayFilters does not match the one in the update query.

D. MongoDB does not support arrayFilters in updateOne.

Solution

1. Step 1: Check identifier consistency

   The update uses $[i] but the arrayFilters uses item. These must match exactly.

2. Step 2: Understand effect of mismatch

   Because the identifiers differ, MongoDB cannot apply the filter, so no elements are updated.

3. Final Answer:

   The identifier in arrayFilters does not match the one in the update query. -> Option C

4. Quick Check:

   Identifier names must match in update and arrayFilters [OK]

Hint: Match identifier names exactly in update and arrayFilters [OK]

Common Mistakes:

• Using different names for identifier and filter variable
• Assuming $set can't work with $[identifier]
• Blaming query filter without checking arrayFilters

5. You have documents with an array field tasks where each task has status and priority. You want to update only tasks with status: 'pending' and priority greater than 5, setting their status to 'in-progress'. Which update command correctly does this?

hard

A. db.collection.updateMany({}, { $set: { 'tasks.$.status': 'in-progress' } }, { arrayFilters: [ { 't.status': 'pending', 't.priority': { $gt: 5 } } ] })

B. db.collection.updateMany({}, { $set: { 'tasks.$[t].status': 'in-progress' } }, { arrayFilters: [ { 't.status': 'pending', 't.priority': { $gt: 5 } } ] })

C. db.collection.updateMany({}, { $set: { 'tasks.$[t].status': 'in-progress' } }, { arrayFilters: [ { 't.status': 'pending' && 't.priority': { $gt: 5 } } ] })

D. db.collection.updateMany({}, { $set: { 'tasks.$[t].status': 'in-progress' } }, { arrayFilters: [ { 't.status': 'pending' }, { 'u.priority': { $gt: 5 } } ] })

Solution

1. Step 1: Use a single filter object combining conditions

   The arrayFilters option must have one object combining both conditions with commas, meaning AND logic.

2. Step 2: Use correct identifier and update syntax

   The update uses $[t] matching the filter identifier t, and $set to update the status field.

3. Final Answer:

   db.collection.updateMany({}, { $set: { 'tasks.$[t].status': 'in-progress' } }, { arrayFilters: [ { 't.status': 'pending', 't.priority': { $gt: 5 } } ] }) -> Option B

4. Quick Check:

   Combine conditions in one filter object with matching identifier [OK]

Hint: Combine multiple conditions in one arrayFilter object [OK]

Common Mistakes:

• Splitting conditions into multiple filter objects (no AND)
• Using logical operators incorrectly inside arrayFilters
• Using positional $ instead of filtered $[identifier]