media_assets.rb

class Externalsource
  include MongoMapper::Document
  one :media_asset, :as => :icon
end

class List
  include MongoMapper::Document
  one :media_asset, :as => :icon
end

# Assets

class MediaAsset
  include MongoMapper::Document
  many :device_specific_assets
end

class DeviceSpecificAsset < CloudAsset
  include MongoMapper::EmbeddedDocument # to illustrate
  belongs_to :device_profile
  
  def for_device(device_profile)
    device_assets.where(:device_profile => device_profile)
  end
end