CartItemInterface cannot add extension_attributes

[degaray]

I am trying to extend CartItemInterface with custom attributes. It goes well, in most parts, but it crashes when calling /carts/mine/payment-information. This is the error I get:

  "message": "Recoverable Error: Argument 1 passed to Magento\Quote\Model\Cart\Totals\Item::setExtensionAttributes() must be an instance of Magento\Quote\Api\Data\TotalsItemExtensionInterface, instance of Magento\Quote\Api\Data\CartItemExtension given, called in /vagrant/data/magento2/lib/internal/Magento/Framework/Api/DataObjectHelper.php on line 125 and defined in /vagrant/data/magento2/app/code/Magento/Quote/Model/Cart/Totals/Item.php on line 470",
  "trace": "#0 /vagrant/data/magento2/app/code/Magento/Quote/Model/Cart/Totals/Item.php(470): 

[mitul69]

hey @degaray

Have you added dependency in your di.xml file?

[ishakhsuvarov]

@degaray Thank you for reporting. We already have a similar item in our backlog (internal ticket MAGETWO-50025) to investigate this.

[degaray]

Thanks. Do you have a workaround?

[ishakhsuvarov]

@degaray I'm afraid we don't have one currently. We will update this ticket as soon as some more information is available.
Thank you.

[degaray]

Ok, here is what I have found so far. If you create an extension_attribute for Magento\Quote\Api\Data\CartItemInterface it works well. However when you call /carts/mine/payment-information Magento tries to convert the CartItemInterface into a Magento\Quote\Model\Cart\Totals\TotalsItemInterface. The problem is that at certain point in the conversion specifically in Magento\Framework\Api\DataObjectHelper::_setDataValues (125) it tries to run TotalItem's setExtensionAttribute, and it tries to set it with CartItem's extension_attributes. Since CartItem's extension_attributes has a different type than TotalItem's it gives the exception.
My best guess is you should convert the extension_attributes type in Magento\Quote\Model\Cart\CartTotalRepository::get before populateWithArray.

[ishakhsuvarov]

@degaray Closing this ticket, as the fix seems to be delivered with the linked commit. Please feel free to reopen if you have any additional questions.

[magento-team]

Internal ticket to track issue progress: MAGETWO-69223