Calculation: updating a field based on Values in another Field

[meks285]

Hi,

 

I have a customers table, products table and an Invoice table.

 

The Products table has a Stock Field(product quatity in Stock).

The Invoice table has a Quantity Field (Quantity purchased by the Customer)

 

I want the Stock field for each product to Decrease in value as the Quantity field increases for every purchase made.

 

Can I get help with this?

 

I am new with the FileMaker Pro

 

Thank you.

[meks285]

I have been able to Work around this by Creating a new Field called remaining which calculates (Stock - Invoice::Quantity) but the calculation only affect the First row in the Invoice table, I will like this to affect all the records in the invoice table.

 

Thank you.