pizza_order_total_price.py

    @property
    def total_price(self) -> Decimal:
        """
        Total price is sum of the following:
            - Pizza total price
            - Size price
            - Sum of toppings 
        """
        price = self.pizza.total_price + self.size.price
        for topping in self.extra_toppings.all():
            price += topping.price

        return price