vince_serializer_example.rb

class AdsSerializer < ActiveModel::Serializer
  attributes :target_url,
             :image_url,
             :thumb_url
  def image_url
    image.url
  end
  def thumb_url
    thumb.url
  end
end

class AndroidAdSerializer < AdsSerializer
  attributes :something_custom
end

# in the controller
if @i_know_its_android
  render json: @ad, serializer: AndroidAdSerializer
else
  render json: @ad
end