在PrestaShop中获取产品的组合属性

I am trying to display Product's combination attributes (like "size") in the front-office of PrestaShop.

When I say "combination attributes" I mean this (selected Size : S, for example):

Size : S

I tried doing it this way:

Code sample

But all I get is this:

enter image description here

There is no "size" key in the results and I am a little bit lost. I am trying to send this data to the CartController (which I will override) and display the selected size in the cart. Can you help me with retrieving this data, please?