Taking ModDate of portal records into consideration

In FileMaker, how do you get the “real” modification date of a record, if it has related records. E.g. if you have a sale, which has a mod date field, and line items for that sale, the mod date for the sale itself does not get changed when the portal records get changed. Solution: Add a “mod date” field to the line items, and add a calc field (type date) to the main record which is something like: Max(Max(Product Line Items::Date Modified), Date Modified)