Only nullify persisted has_one target associations

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.

1 files changed, 1 insertions, 0 deletions

diff --git a/activerecord/test/models/developer.rb b/activerecord/test/models/developer.rb
index d2a5a7fc49..8ac7a9df6a 100644
--- a/activerecord/test/models/developer.rb
+++ b/activerecord/test/models/developer.rb
@@ -50,6 +50,7 @@ class Developer < ActiveRecord::Base
   has_many :firms, :through => :contracts, :source => :firm
   has_many :comments, ->(developer) { where(body: "I'm #{developer.name}") }
   has_many :ratings, through: :comments
+  has_one :ship, dependent: :nullify
 
   scope :jamises, -> { where(:name => 'Jamis') }