I suggest you start with the online learning path for developers: https://academy.mendix.com/link/path/38/Become-a-Rapid-Developer-(Developer)
This will give you a head start. Where you will learn that associations are not define by keys you define yourself, but are auto-generated & handled by Mendix. This will certainly help you with you challenge.
GoLearnIt!
Thanks, Rene. I'll do it