How do I set a belongs_to association to Nil when the associated object is destroyed

Say I have: ``` class Car < ActiveRecord::Base belongs_to :brand end class Brand < ActiveRecord::Base has_many :cars end ``` If I destroy a Brand object, the child Cars still have their `car.brand_id` attributes set to the destroyed `brand.id`. How can I null out the `car.brand_id` of the child Cars when destroying the parent Brand? I thought the ActiveRecord relationship would handle this, and prevent orphaned objects. I don't want to `dependent: :destroy` the Cars, but just want to have their `car.brand_id` set to nil. I don't want to have to write an `after_commit` for this, I want Rails magic to handle this. Or less ideally a foreign key constraint.