class ActiveModel::Serializer::Associations::HasOne

def serialize_ids
  if polymorphic?
    if associated_object
      {
        :type => polymorphic_key,
        :id => associated_object.read_attribute_for_serialization(embed_key)
      }
    else
      nil
    end
  elsif !option(:embed_key) && source_serializer.object.respond_to?("#{name}_id")
    source_serializer.object.send("#{name}_id")
  elsif associated_object
    associated_object.read_attribute_for_serialization(embed_key)
  else
    nil
  end
end