Array update with $[identifier] filtered in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

It allows you to update specific elements in an array that match a filter condition, using an identifier to refer to those elements.

intermediate

How do you specify which array elements to update when using $[identifier]?

You define an arrayFilters option with a condition that matches the elements you want to update, and use the same identifier in the update statement.

intermediate

Example: Update all array elements with status: 'pending' to status: 'complete' using $[elem]. What is the key part of the update query?

Use { $set: { 'arrayField.$[elem].status': 'complete' } } with arrayFilters: [{ 'elem.status': 'pending' }].

advanced

Can you use multiple $[identifier] filters in one update?

Yes, you can use multiple identifiers with different conditions in arrayFilters to update different parts of arrays in one command.

intermediate

Why is $[identifier] useful compared to positional operator $?

$[identifier] lets you update multiple array elements matching a condition, while $ updates only the first matching element.

What does $[elem] represent in a MongoDB update?

A A new array to replace the old one

B A placeholder for filtered array elements to update

C The entire array field

D A query operator to delete elements

$[elem] is used to target specific array elements that match a filter condition for update.

How do you specify which elements $[identifier] refers to?

A Using <code>projection</code> in the query

B Using <code>limit</code> option

C Using <code>sort</code> option

D Using <code>arrayFilters</code> option with conditions

The arrayFilters option defines conditions for elements matched by $[identifier].

What happens if you use $[identifier] without arrayFilters?

A The update will fail with an error

B All array elements will be updated

C Only the first element will be updated

D The array will be deleted

MongoDB requires arrayFilters to define the filter for $[identifier], otherwise it errors.

Which operator updates only the first matching array element?

A $

B $[identifier]

C $set

D $push

The positional operator $ updates only the first matching element in an array.

Can you update multiple different arrays in one update using multiple $[identifier] filters?

A Yes, but only if arrays have the same name

B No, only one array can be updated at a time

C Yes, with multiple arrayFilters and identifiers

D No, you must run separate updates

MongoDB allows multiple $[identifier] filters with corresponding arrayFilters to update multiple arrays in one command.

Explain how to update specific elements in a MongoDB array using $[identifier] and arrayFilters.

Think about how to target only some items inside an array.

Describe the difference between the positional operator $ and the filtered positional operator $[identifier] in MongoDB updates.

Consider how many elements each operator can update.

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]