fog_file.rb

class FogFile
  def initialize(path)
    @path = path
  end

  def url
    directory = connection.directories.new(:key => uploader.fog_directory)
    file = directory.files.new(:key => @path)
    file.url(::Fog::Time.now + uploader.fog_authenticated_url_expiration)
  end

  def connection
    @connection ||= CarrierWave::Storage::Fog.new(uploader).connection
  end

  def uploader
    @uploader ||= OpenStruct.new(
      :fog_authenticated_url_expiration => 600,
      :fog_credentials => {
        :provider               => 'AWS',
        :aws_access_key_id      => ENV['S3_ACCESS_KEY_ID'],
        :aws_secret_access_key  => ENV['S3_SECRET_ACCESS_KEY'],
        :region                 => ENV['S3_REGION'] || 'eu-west-1'
      },
      :fog_directory => ENV['S3_BUCKET_NAME']
    )
  end
end

product_seed.rb

Product.all.each do |product|
  product.update_attributes(:remote_factsheet_url => FogFile.new("products/#{product.identifier}.pdf").url)
end