after_add callback for has_one

This seems to be an inconsistency between has_many and has_one.

The has_many association allows you to specify an after_add callback that is called after an object has been added to the collection.

class Person
  has_many :parents, :after_add => { puts "Added new parent" } # allowed
  has_one :car, :after_add => { puts "Added car" } # not allowed
end

class Car
  after_create :assign_name
  def assign_name
    self.name = "Herbie"
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end

Unfortunately, there isn't an after_add callback for the has_one association. How do you achieve the same thing for a has_one?

I think you can use before_save, and check if the car relation has changed:

before_save :do_something

def do_something
  puts "Added car" if car_changed?
end