postgres function for loop - sql

Question

I have an orders table and an order_products_items table.
The order_products_items has these fields:
order_id
product_id
quantity
price
I am trying to create a calculated_field: calculated_total_products_price in the orders table through a before insert trigger function that would calculate the total order price by looping through all the order_products_items related to the order and multiplying the quantity with the price for every order item.
This is my failed attempt in doing so:
CREATE OR REPLACE FUNCTION public.fn_trigger_total_order_price()
RETURNS trigger
LANGUAGE plpgsql
AS $function$
declare total float := 0.0;
product record;
BEGIN
FOR product IN
SELECT itm.price, itm.quantity
FROM order_products_items itm
INNER JOIN orders ord
ON itm.order_id = ord.id
WHERE ord.id = NEW.id
LOOP
total = total + (product.price * product.quantity);
END LOOP;
NEW.calculated_total_products_price := total;
RETURN NEW;
END;
$function$
;
The trigger looks like this:
CREATE TRIGGER fn_trigger_total_order_price BEFORE INSERT ON public.orders
FOR EACH ROW EXECUTE PROCEDURE fn_trigger_total_order_price();
Somehow, I do not get any errors, but always get 0 as a result.
Am I missing something? or is there a better/more efficient way of approaching this?
Many thanks.

Answer 1

You don't need a loop in postgres to achieve this.
A simple AGGREGATION query would do.
Something like
SELECT SUM(quantity * price)
FROM order_products_items
WHERE order_id = NEW.id
You also don't really need to mess with a trigger if you don't want to.
Just make a view out of the query below and relate it to the orders table as something like order_totals
SELECT order_id, SUM(quantity * price) order_total
FROM order_products_items
GROUP BY order_id