Ruby on Rails - Run after_save only if new records

In my application I have models Post & Image. My association are:

class Post < ActiveRecord::Base
  has_many :images
  accepts_nested_attributes_for :images, reject_if: :image_rejectable?, allow_destroy: true

class Image < ActiveRecord::Base
  belongs_to :post

I use cocoon gem for nested_forms

When user is adding images, I have some global settings that user can apply to images they are adding.

I do it by doing:

class Post < ActiveRecord::Base
  has_many :images
  accepts_nested_attributes_for :images, reject_if: :image_rejectable?, allow_destroy: true

  after_create :global_settings

  private

    def global_settings
      self.images.each { |image| image.update_attributes(
                            to_what: self.to_what,
                            added_to: self.added_to,
                            )
                      }
    end

This works fine, but now I want it so if they want to edit post's images, I want to apply same post global settings ONLY to new records.

I tried to do it by doing:

class Post < ActiveRecord::Base
  has_many :images
  accepts_nested_attributes_for :images, reject_if: :image_rejectable?, allow_destroy: true

  after_save :global_settings

  private

  def global_settings
    if new_record?
      self.images.each { |image| image.update_attributes(
          to_what: self.to_what,
          added_to: self.added_to,
      )
      }
    end
  end

This didn't worked at all & global setting were not added to any record (nor on new/create or edit/update action).

I've also tried with:

after_save :global_settings, if: new_record?

This gave me error: undefined method 'new_record?' for Post

How can I only apply my global settings for all new records/new images?

ps: I tried to find some answer on SO, but nothing worked!

Since images doesn't have those global settings means that you can only execute the function only on images that doesn't have all fields.

def global_settings
  self.images.each { |image|
    if image.to_what.blank?
      image.update_attributes(
          to_what: self.to_what,
          added_to: self.added_to
      )
    end
  }
end