I have two models (Car and Driver) and a combobox to select which driver
belongs to which car. I want the combobox just show the objects that
not yet been associated.
vehicle belongs_to: driver
driver has_one: vehicle
simple_form # vehicle/_form.html.haml
= f.association: driver, label_method: :name, value_method: :id
How to ensure validation before saving, to avoid problems of concurrent