[5.x]: LineItems aren't updated with update cart action cart/update-cart

[roguesherlock]

What happened?

Description

When updating a cart, the line item is expected to update based on the id provided in the payload. However, the update fails because the code appears to use the array index instead of the provided id, preventing the correct line item from being updated.

Steps to reproduce

1. Send a request to update the cart with the following JSON payload:

   {
       "lineItems": [{
           "id": 129,
           "qty": 10
       }]
   }

2. Check the cart to see if the line item with id 129 has been updated.

Expected behavior

The cart should update the line item with the provided id (in this case, 129), so that the line item reflects the new quantity. In this case, the updated line item should show a quantity of 10.

Actual behavior

The cart message says cart updated but the line item is not updated.

I dug through the code a bit I see that it attempts to use the array index (0) as the line item ID instead of using the id field (129) from the line item object, causing the wrong item to be fetched or no item to be found.

commerce/src/controllers/CartController.php

Lines 245 to 246 in 75832ab

	foreach ($lineItems as $key => $lineItem) {
	$lineItem = $this->_getCartLineItemById($key);

if I send the line Items as an object of key (id) /value (lineItem) pairs like the code expects it seems to work

   {
      "lineItems": {
        "129": {
            "id": 129,
            "qty": 10
        }
      }
   }

However the documentation states

Array of one or more of the cart's line items to update. Each must have an id key-value pair, and may include options, note, and qty key-value pairs.

I'm not sure if it's the expected behaviour and the documentation is incorrect (or I misunderstood it) or it's actually a bug.

Craft CMS version

Craft Pro 5.6.4

Craft Commerce version

5.3.1

PHP version

8.3.14

Operating system and version

Darwin 24.3.0

Database type and version

MySQL 8.4.4

Image driver and version

Imagick 3.7.0 (ImageMagick 7.1.1-41)

Installed plugins and versions

[linear]

PT-2453 [5.x]: LineItems aren't updated with update cart action `cart/update-cart`