i am selling tickets for a raffle as a fundraiser for a non-profit and I want to either allow users to request a set of ticket numbers that have not yet been sold, or let the form assign them numbers that haven't been issued yet. If the user asks for ticket with numbers that have already been sold, that would not be allowed. I would create a form that asks the user how many tickets they want, and then has a radio button to either auto-assign ticket numbers or let the user request specific ticket numbers (e.g., their "lucky numbers"). If the user requests specific ticket numbers, I would have them enter a starting and ending number.  There would be a view available showing the list of ticket numbers already purchased to let people know what's already sold.  But inevitably, someone will try to buy tickets that have already been sold.  How would I go about checking to make sure the numbers requested were still available?  I would want the form to pop up with an error message saying those tickets have already been sold, please check the view and try again. Does anyone have a suggested approach to accomplish this?