class ActiveModel::Serializers::Xml::Serializer

:only is set, always delete :except.
level model can have both :except and :only set. So if
then because :except is set to a default value, the second
for a N level model but is set for the N+1 level models,
takes precedence over :only. If :only is not set
To replicate the behavior in ActiveRecord#attributes, :except

def attributes_hash
  attributes = @serializable.attributes
  if options[:only].any?
    attributes.slice(*options[:only])
  elsif options[:except].any?
    attributes.except(*options[:except])
  else
    attributes
  end
end