Server needs to be restarted when using Para::Sti::RootModel

[gabriel-cardoso]

When using a Para::Sti::RootModel in a nested model, the dropdown with the subclasses is empty when I edit some code in my app. Server needs to be restarted to correctly load the subclasses list.

[vala]

This was fixed when we added the :nested_many input the :subclasses option, which forces us to explicitly add subclasses to the dropdowns, and fixes this issue which was bound to Rails dev environment reloading strategy