How to has_one/belongs_to association only return the objects unassociated in combobox?

Rails
1

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

2

As per readme of simple_form
(GitHub - heartcombo/simple_form: Forms made easy for Rails! It's tied to a simple DSL, with no opinion on markup.),
the f.association helper is available with the collection option which
accepts the collection of objects that you want to provide for select
list.
You can use :

f.association :driver, collection: Driver.unassociated, label_method:
:name, value_method: :id

In Driver modelclass Driver < ActiveRecord::Base

scope :unlinked, → { where(car_id: nil) }
end

How to ensure validation before saving, to avoid problems of concurrent
accesses?

For validation you can use uniqueness validation as :

class Driver < ActiveRecord::Base

validates_uniqueness_of :car_id
end

This way when any object of Driver is saved it will be checked that
weather
the car_id was taken earlier or not and so the same car_id will not be
assigned to more than one driver.
Thanks,
Lauree