preload_one_association

# File activerecord/lib/active_record/association_preload.rb, line 110
      def preload_one_association(records, association, preload_options={})
        class_to_reflection = {}
        # Not all records have the same class, so group then preload
        # group on the reflection itself so that if various subclass share the same association then
        # we do not split them unnecessarily
        records.group_by { |record| class_to_reflection[record.class] ||= record.class.reflections[association]}.each do |reflection, _records|
          raise ConfigurationError, "Association named '#{ association }' was not found; perhaps you misspelled it?" unless reflection

          # 'reflection.macro' can return 'belongs_to', 'has_many', etc. Thus,
          # the following could call 'preload_belongs_to_association',
          # 'preload_has_many_association', etc.
          send("preload_#{reflection.macro}_association", _records, reflection, preload_options)
        end
      end