module ActiveRecord
  module Associations
    class Preloader
      module ThroughAssociation #:nodoc:
        def through_reflection
          reflection.through_reflection
        end

        def source_reflection
          reflection.source_reflection
        end

        def associated_records_by_owner(preloader)
          preloader.preload(owners,
                            through_reflection.name,
                            through_scope)

          through_records = owners.map do |owner|
            association = owner.association through_reflection.name

            center = target_records_from_association(association)
            [owner, Array(center)]
          end

          reset_association owners, through_reflection.name

          middle_records = through_records.flat_map { |(_, rec)| rec }

          preloaders = preloader.preload(middle_records,
                                         source_reflection.name,
                                         reflection_scope)

          @preloaded_records = preloaders.flat_map(&:preloaded_records)

          middle_to_pl = preloaders.each_with_object({}) do |pl, h|
            pl.owners.each { |middle|
              h[middle] = pl
            }
          end

          through_records.each_with_object({}) do |(lhs, center), records_by_owner|
            pl_to_middle = center.group_by { |record| middle_to_pl[record] }

            records_by_owner[lhs] = pl_to_middle.flat_map do |pl, middles|
              rhs_records = middles.flat_map { |r|
                association = r.association source_reflection.name

                target_records_from_association(association)
              }.compact

              # Respect the order on `reflection_scope` if it exists, else use the natural order.
              if reflection_scope.values[:order].present?
                @id_map ||= id_to_index_map @preloaded_records
                rhs_records.sort_by { |rhs| @id_map[rhs] }
              else
                rhs_records
              end
            end
          end
        end

        private

          def id_to_index_map(ids)
            id_map = {}
            ids.each_with_index { |id, index| id_map[id] = index }
            id_map
          end

          def reset_association(owners, association_name)
            should_reset = (through_scope != through_reflection.klass.unscoped) ||
               (reflection.options[:source_type] && through_reflection.collection?)

            # Don't cache the association - we would only be caching a subset
            if should_reset
              owners.each { |owner|
                owner.association(association_name).reset
              }
            end
          end

          def through_scope
            scope = through_reflection.klass.unscoped

            if options[:source_type]
              scope.where! reflection.foreign_type => options[:source_type]
            else
              unless reflection_scope.where_clause.empty?
                scope.includes_values = Array(reflection_scope.values[:includes] || options[:source])
                scope.where_clause = reflection_scope.where_clause
              end

              scope.references! reflection_scope.values[:references]
              if scope.eager_loading? && order_values = reflection_scope.values[:order]
                scope = scope.order(order_values)
              end
            end

            scope
          end

          def target_records_from_association(association)
            association.loaded? ? association.target : association.reader
          end
      end
    end
  end
end