ActiveAdmin и Carrierwave с :has_many

Я сейчас совсем схожу с ума.

У меня есть 2 модели: проект и скриншот:

create_table "projects", :force => true do |t|
  t.string   "name"
  t.text     "description"
  t.boolean  "isactive"
  t.datetime "created_at",  :null => false
  t.datetime "updated_at",  :null => false
  t.string   "slug"
  t.string   "logo"
  t.string   "teaser"
end

add_index "projects", ["slug"], :name => "index_projects_on_slug"

create_table "screenshots", :force => true do |t|
  t.integer  "project_id"
  t.string   "image"
  t.datetime "created_at", :null => false
  t.datetime "updated_at", :null => false
end

Модель проекта выглядит так

class Project < ActiveRecord::Base

  attr_accessible :name, :description, :isactive, :slug, :logo, :teaser, :screenshots_attributes

  scope :isactive, :conditions => ["isactive = ?",true]

  mount_uploader :logo, LogoUploader

  extend FriendlyId
  friendly_id :name, use: [:slugged, :history]

  has_many :screenshots
  accepts_nested_attributes_for :screenshots

end

и модель на скриншоте

class Screenshot < ActiveRecord::Base

  belongs_to :project, :polymorphic => true
  mount_uploader :screenshots, ScreenshotUploader

end

Загрузчик скриншотов в настоящее время не редактируется:

# encoding: utf-8

class ScreenshotUploader < CarrierWave::Uploader::Base

  # Include RMagick or MiniMagick support:
  # include CarrierWave::RMagick
  # include CarrierWave::MiniMagick

  # Choose what kind of storage to use for this uploader:
  storage :file
  # storage :fog

  # Override the directory where uploaded files will be stored.
  # This is a sensible default for uploaders that are meant to be mounted:
  def store_dir
    "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end

  # Provide a default URL as a default if there hasn't been a file uploaded:
  # def default_url
  #   "/images/fallback/" + [version_name, "default.png"].compact.join('_')
  # end

  # Process files as they are uploaded:
  # process :scale => [200, 300]
  #
  # def scale(width, height)
  #   # do something
  # end

  # Create different versions of your uploaded files:
  # version :thumb do
  #   process :resize_to_fit => [300, 150]
  # end

  # Add a white list of extensions which are allowed to be uploaded.
  # For images you might use something like this:
  # def extension_white_list
  #   %w(jpg jpeg gif png)
  # end

  # Override the filename of the uploaded files:
  # Avoid using model.id or version_name here, see uploader/store.rb for details.
  # def filename
  #   "something.jpg" if original_filename
  # end

end

А сейчас пытаюсь добавить скрины из activeadmin через эту форму

form do |f|
  f.inputs "Project Details" do
    f.input :name
    f.input :logo, :as => :file, :hint => f.template.image_tag(f.object.logo.url)
    f.input :teaser
    f.input :description
    f.input :isactive
    f.has_many :screenshots do |s|
      s.input :image, :as => :file
    end
  end
  f.buttons
end

Но все, что я получаю, это:

NoMethodError (undefined method `screenshots_changed?' for #<Screenshot:0xc940cd0>):

Я играл с этим последние часы, но ничего из того, что я пробовал, не сработало :(

Какие-либо предложения?