order_discount_price not saved to db when using discount - only with coupons

Hi,

The coupons are applied globally to the cart/order.
The discounts are applied to the product prices directly. So in the order you see the product prices already with the discount applied.
Storing the discount amount in order_discount_price is not possible. Imagine if you have two discounts on two different products of the order.
Even if we were to store the data of the discounts with the order, they wouldn't be stored there. They would be stored in the hikashop_order_product table along with the products on which they apply.
The name of the columns in the database use "discount" because the coupons are actually also stored in the hikashop_discount table, but that's all.

Since the discount information is not stored with the orders for now, the best would be to implement the onAfterOrderCreate trigger in your plugin in order to get the data of the discounts in the products from the $order->cart->products array and store them yourself in some column of the hikashop_order_product table.
In fact that's something we should do in a future release of HikaShop.

You can create custom fields of the table "item" via the menu Display>Custom fields.

Hi,

In the onBeforeOrderCreate function of your plugin, you can simply add the data of your custom item field to each product object in the array $order->cart->products using the column name of the custom field as attribute of the product object.
Note that the "frontend" display setting of the custom field needs to be activated or the field will be skipped by HikaShop.