Question

I’ve updated my server to ruby 1.9.2 and this stopped working (rails 3.0.6):

def index  
  @musicians = Musician.includes(:instruments)
  render :xml => @musicians.to_xml( :include => :instruments )
end

And the models:

class Musician < ActiveRecord::Base
  has_and_belongs_to_many :instruments
end

class Instrument < ActiveRecord::Base
  has_and_belongs_to_many :musicians
end

I’m getting this error:

undefined method `type' for nil:NilClass

Framework trace:

activesupport (3.0.6) lib/active_support/whiny_nil.rb:48:in `method_missing'
activerecord (3.0.6) lib/active_record/serializers/xml_serializer.rb:230:in `compute_type'
activemodel (3.0.6) lib/active_model/serializers/xml.rb:22:in `initialize'
activemodel (3.0.6) lib/active_model/serializers/xml.rb:75:in `new'
activemodel (3.0.6) lib/active_model/serializers/xml.rb:75:in `block in serializable_attributes'

Any clue what I’m doing wrong?

Maybe this is related to: https://rails.lighthouseapp.com/projects/8994/tickets/4840-to_xml-doesnt-work-in-such-case-eventselecttitle-as-tto_xml

Answer 1

This is a core issue with Rails. What’s happening is that when Instruments are being included, an instrument_id attribute is getting added. Then, when each Instrument is serialized, the XmlSerializer class determines the type of that attribute based on the Instrument class’ definition, using the type attribute for each column. Since the instrument_id attribute does not exist in the class definition, a nil object is returned which, as of Ruby 1.9, does not have a type attribute, which Rails is depending on.

(I don’t think the patch in the thread you linked to works — but the one I’ve provided below does.)

There are two ways to fix this:

1. Don’t serialize instrument_id (good idea).

   render :xml => @musicians.to_xml( :include => { :instruments => { :except => :instrument_id } } )

2. Or patch Rails core (bad idea).

   --- a/activerecord/lib/active_record/serializers/xml_serializer.rb  2011-04-20 15:01:10.000000000 -0700
   +++ b/activerecord/lib/active_record/serializers/xml_serializer.rb      2011-04-20 15:00:42.000000000 -0700
   @@ -226,8 +226,10 @@
   
        class Attribute < ActiveModel::Serializers::Xml::Serializer::Attribute #:nodoc:
          def compute_type
   +        Rails.logger.info("key: #{name}, hash: #{@serializable.class.columns_hash[name]}")
            type = @serializable.class.serialized_attributes.has_key?(name) ?
   -          super : @serializable.class.columns_hash[name].type
   +          super : @serializable.class.columns_hash[name].nil? ?
   +            NilClass : @serializable.class.columns_hash[name].type
   
            case type
            when :text