Make changes to list with specific data on each row(?)


I’ve been trying to solve this problem for weeks now. I’m not very good at working with lists, that’s why I have to ask you guys for help.

I’m building a lunch delivery site. I have the following data types: DAY, COURSE, PRODUCT and BASKET.

On the type DAY, there’s a field of type COURSE and SALE which is a number.
The PRODUCT a field of DAY and AMOUNT of how many meals (COURSE).
The BASKET, a list of PRODUCTS.

What I want to do is, at the checkout, make changes to all the PRODUCT’S DAYS. That is, the AMOUNT on that PRODUCT should be added to that PRODUCT’S DAY’S SALE.

Thank you so much in advance!