Unused potential of OrderItem::$relatedItem

[vitek-rostislav]

Describe the bug

We have $relatedItem property in OrderItem class (https://github.com/shopsys/shopsys/blob/13.0/project-base/app/src/Model/Order/Item/OrderItem.php#L50) but it is basically not used. It is just set when creating a new order item of type coupon (which is actually an order item of type OrderItem::TYPE_PRODUCT at the moment while it deserves its own type, but we all know that 🙂). The getters are then never used if I am not mistaken.

I have two suggestions for how to improve the current implementation:

1. $relatedItem should be rather implemented as one-to-many relation (there might be more related items eg. when selecting a collection of services for a product). Something like this is better for a wider scale of use-cases IMHO:

    /**
     * @var \App\Model\Order\Item\OrderItem|null
     * @ORM\ManyToOne(targetEntity="App\Model\Order\Item\OrderItem", inversedBy="relatedOrderItems")
     * @ORM\JoinColumn(nullable=true)
     */
    private ?OrderItem $parentOrderItem;

    /**
     * @var \Doctrine\Common\Collections\Collection<int, \App\Model\Order\Item\OrderItem>
     * @ORM\OneToMany(targetEntity="App\Model\Order\Item\OrderItem", mappedBy="parentOrderItem")
     */
    private Collection $relatedOrderItems;

2. We should really use the relation when we approach the related items (on the storefront, in the mail, etc.) - at the moment, we just iterate through the items and rely on the fact the related items are created right after their "parent" item - ie. we rely on the following: parentItem->getId() + 1 === $relatedItem->getId(). If we messed up the ids, the output would be broken. Instead, I suggest iterating just the PRODUCT_TYPE items and under them, iterating through their relatedItems to display the related coupons, services, or whatever the related items might be on the particular implementation.
3. BONUS - as mentioned above, there should be a separate type for the OrderItem that represents the discount/coupon. Sad enough, there are two other issues (PromoCodes and OrderRounding are OrderItems with type product #1390 and Promo code is custom instance of OrderItem #1263) and one pull request (Order item has new type discount #1563) for this ☹️