Solved: Update a Sharepoint list with values from another one

I have created a stock monitoring app based on two Sharepoint lists. The first list has stock item (text column) and quantity (number column) data as well as other fields for each item. The second list has stock movements (number column) in it, with columns for item (a Lookup field from the stock item column in the first list), date, stock out quantity (number column) and location (choice column) it is out to.

My app is linked to both lists.

I can view, edit and add new items to the first list and I can view, edit and add new items to the second list.

My question is, what is the best way for me to link the lists? At the moment they act independently. I want the stock out quantity from list 2 to update the relevant stock item's stock quantity in the first list every time a stock item is taken out.

How do I do this with my app? And, is this the best way?

Thanks.

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Connecting to Data

General Questions