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.