UPDATE: A small bug in the project package (the one that Mendix provides as a resource in module 7.1 of the Rapid Developer learning path), I think.
The Add button on the Registration_Overview page does not have the TrainingEvent entity as a constraint (Registration_TrainingEvent/Registration), but uses the Registration entity instead. Hence, a new Registration object is not being associated to a TrainingEvent object and is not shown on the TrainingEvent_Registration_Overview page.
Changing the entity for the Add button on the Registration_Overview page from "Registration" to "Registration_TrainingEvent/Registration" solved it.
Okay, that only took me half a day ... Now finally back to module 7.10.2 and trying to set the total number of registrations on the TrainingEvent_Overview page ;-)
Hello Anne,
I would first check if security is enabled, if it is I would try it with it disabled, second I would check if you have a commit action when you're saving the TrainingEvent objects (not needed if you're using the default save button).
Hope this helps