Field Calculation advice: Adding "Qty" ONLY for records w/ a date entered in "Date Paid"
In an Event Tracking db I email event registration invoices which can be for one or many tickets. When the invoice gets paid I enter the date received in the "Date Paid" fied. When that happens I want the total Qty of tickets on that particular invoice to be added to the "Total Registrations" field but not for records that are still outstanding.
I tried this but it's wrong: If(IsEmpty(Invoice::DatePaid);0;Sum(Event::TotalRegistrations)