Rails - Apr 2014 - How to has_one/belongs_to association only return the objects unassociated in combobox?

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 have
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
accesses?

Thanks

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