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carousel.rb

require 'mongoid'

module GluU
  class Carousel
    include Mongoid::Document
    field :link, type: String
    field :index, type: Integer
    embeds_one :image, class_name: 'GluU::Carousel::Image', cascade_callbacks: true
    delegate :url, to: :background_image, prefix: true

    def background_image=(file)
      background_image.replace file
    end

    def background_image
      image || DefaultBackground.new(self)
    end
 
    def replace_background_image(file)
      build_image.replace file
    end
  end 
end

default_background.rb

module GluU
  class Carousel::DefaultBackground
    attr_accessor :model
    def initialize(model)
      self.model = model
    end
 
    def url
      '/assets/images/example-banner-1-f5d6421c4edb3f22071c9cd30c6c67ea.png'
    end 

    def replace(file)
      self.model.replace_background_image file
    end
  end
end

image.rb

require 'mongoid'
require 'carrierwave'

module GluU
  class Carousel::Image
    class Uploader < CarrierWave::Uploader::Base
      include CarrierWave::ImageScience
      process resize_to_fit: [560, 330]
    end 

    include Mongoid::Document    mount_uploader :file, Uploader
    embedded_in :carousel, class_name: 'Carousel', inverse_of: :image
    delegate :url, to: :file
    
    def replace(file)
      update_attributes file: file
    end
  end
end