def append_to_solr_doc(solr_doc, solr_field_key, field_info, val)
      return super unless object.controlled_properties.include?(solr_field_key.to_sym)
      case val
      when ActiveTriples::Resource
        append_label_and_uri(solr_doc, solr_field_key, field_info, val)
      when String
        append_label(solr_doc, solr_field_key, field_info, val)
      else
        raise ArgumentError, "Can't handle #{val.class}"
      end
    end
def append_to_solr_doc(solr_doc, solr_field_key, field_info, val)
      return super unless object.controlled_properties.include?(solr_field_key.to_sym)
      case val
      when ActiveTriples::Resource
        append_label_and_uri(solr_doc, solr_field_key, field_info, val)
      when String
        append_label(solr_doc, solr_field_key, field_info, val)
      else
        raise ArgumentError, "Can't handle #{val.class}"
      end
    end
We're overiding the default indexer in order to index the RDF labels. In order
 for this to be called, you must specify at least one default indexer on the property.
 @param [Hash] solr_doc
 @param [String] solr_field_key
 @param [Hash] field_info
 @param [ActiveTriples::Resource, String] val