nested_attributes_for_strategy.rb

class NestedAttributesForStrategy
  def association(runner)
    runner.run
  end

  def result(evaluation)
    evaluation.object.tap do |instance|
      evaluation.notify(:after_build, instance)
      return attributes(instance)
    end
  end

  private

  def attributes(instance)
    attrs = instance.attributes.delete_if do |k, _|
      %w(id type created_at updated_at).include?(k)
    end
    nested_reflections(instance).each do |ref|
      attrs.merge!("#{ref.name}_attributes" => instance.send(ref.name).map do |nested_obj|
        attributes(nested_obj)
      end)
    end
    attrs
  end

  def nested_reflections(instance)
    instance.reflections.values.select do |ref|
      ref.macro == :has_many && instance.respond_to?("#{ref.name}_attributes=")
    end
  end
end