Hi Iulia Triff
Mendix will not support AutoNumber for Offline profile and for non-persistent entity.
Incase if you want to us the auto number function have an attribute in the parent entity as count and increment is always and assign it to child entity.
For example: Taking example of the Training Registration app that you are working!
From the screenshot I see that you are trying to show the list of Registration
And let’s refer the
Registration entity = Child entity
TrainingEvent entity = Parent entity
1 Training Event has * Registrations
Maintain an attribute in the TrainingEvent entity. Always increase the attribute with +1 while after creating the Registration entity.
That will be similar to auto number functionality!
Hope it help!