mark as deleted idea

models.rb

class Image
  def mark_as_deleted(build = nil)
    # If there's only one undeleted build left, we should only delete the one passed in
    # otherwise, we're free to delete them all, and we should delete self as well.
    if build && self.has_more_than_one_undeleted_build?
      build.mark_as_deleted
    else
      build.each(&:mark_as_deleted)
      self.update_column :deleted_at, Time.current
    end
  end
end

class Build
  # @api: private
  def mark_as_deleted
    self.update_column :deleted_at, Time.current
  end
end

no_api_change.rb

class Image
  def mark_as_deleted
    not_deleted_builds.each(&:mark_as_deleted)
    self.update_column :deleted_at, Time.current
  end
end

class Build
  # @api: private
  def mark_as_deleted
    self.update_column :deleted_at, Time.current
    image.mark_as_deleted if image.has_no_builds_left?
  end
end

service_class.rb

class Image
  def mark_as_deleted(build = nil)
    self.update_column :deleted_at, Time.current
    # and do other stuff, but don't call Build#mark_as_deleted
  end
end

class Build
  def mark_as_deleted
    self.update_column :deleted_at, Time.current
    # and do other stuff, but don't call Image#mark_as_deleted
  end
end


class DeletesImagesAndBuilds # You come up with a better name, it's a service class
  def self.delete_image(image)
    image.builds.each(&:mark_as_deleted)
    image.mark_as_deleted
  end

  def self.delete_build(build)
    build.mark_as_deleted
    if build.image.has_no_builds_left?
      image.mark_as_deleted
    end
  end
end

Just a reminder: I don't know the full scope of the issue, this is one idea