set_association_single_records

# File activerecord/lib/active_record/association_preload.rb, line 149
      def set_association_single_records(id_to_record_map, reflection_name, associated_records, key)
        seen_keys = {}
        associated_records.each do |associated_record|
          #this is a has_one or belongs_to: there should only be one record.
          #Unfortunately we can't (in portable way) ask the database for
          #'all records where foo_id in (x,y,z), but please
          # only one row per distinct foo_id' so this where we enforce that
          next if seen_keys[associated_record[key].to_s]
          seen_keys[associated_record[key].to_s] = true
          mapped_records = id_to_record_map[associated_record[key].to_s]
          mapped_records.each do |mapped_record|
            association_proxy = mapped_record.send("set_#{reflection_name}_target", associated_record)
            association_proxy.__send__(:set_inverse_instance, associated_record, mapped_record)
          end
        end

        id_to_record_map.each do |id, records|
          next if seen_keys.include?(id.to_s)
          records.each {|record| record.send("set_#{reflection_name}_target", nil) }
        end
      end