Since after 87d1aba3c `dependent: :destroy` callbacks on has_one
assocations run *after* destroy, it is possible that a nullification is
attempted on an already destroyed target:
class Car < ActiveRecord::Base
has_one :engine, dependent: :nullify
end
class Engine < ActiveRecord::Base
belongs_to :car, dependent: :destroy
end
> car = Car.create!
> engine = Engine.create!(car: car)
> engine.destroy! # => ActiveRecord::ActiveRecordError: cannot update a
> destroyed record
In the above case, `engine.destroy!` deletes `engine` and *then* triggers the
deletion of `car`, which in turn triggers a nullification of `engine.car_id`.
However, `engine` is already destroyed at that point.
Fixes#21223.
19b168e611