preload_belongs_to_association

# File activerecord/lib/active_record/association_preload.rb, line 192
      def preload_belongs_to_association(records, reflection, preload_options={})
        options = reflection.options
        primary_key_name = reflection.primary_key_name

        if options[:polymorphic]
          polymorph_type = options[:foreign_type]
          klasses_and_ids = {}

          # Construct a mapping from klass to a list of ids to load and a mapping of those ids back to their parent_records
          records.each do |record|
            if klass = record.send(polymorph_type)
              klass_id = record.send(primary_key_name)
              if klass_id
                id_map = klasses_and_ids[klass] ||= {}
                id_list_for_klass_id = (id_map[klass_id.to_s] ||= [])
                id_list_for_klass_id << record
              end
            end
          end
          klasses_and_ids = klasses_and_ids.to_a
        else
          id_map = {}
          records.each do |record|
            key = record.send(primary_key_name)
            if key
              mapped_records = (id_map[key.to_s] ||= [])
              mapped_records << record
            end
          end
          klasses_and_ids = [[reflection.klass.name, id_map]]
        end

        klasses_and_ids.each do |klass_and_id|
          klass_name, id_map = *klass_and_id
          klass = klass_name.constantize

          table_name = klass.quoted_table_name
          primary_key = klass.primary_key
          conditions = "#{table_name}.#{primary_key} IN (?)"
          conditions << append_conditions(options, preload_options)
          associated_records = klass.find(:all, :conditions => [conditions, id_map.keys.uniq],
                                          :include => options[:include],
                                          :select => options[:select],
                                          :joins => options[:joins],
                                          :order => options[:order])
          set_association_single_records(id_map, reflection.name, associated_records, primary_key)
        end
      end