Preload with Rails 6+

preload_association.rb

    def preload_association(records)
      ::ActiveRecord::Associations::Preloader.new.preload(
        records,
        @association_schema,
        @preload_scope
      ).then(&:first).then do |preloader|
        next unless @preload_scope
        # The result of previous preload is memoized, ActiveRecord won't load this association again.
        if preloader.is_a?(::ActiveRecord::Associations::Preloader::AlreadyLoaded)
          owner = preloader.send(:owners).first
          # We can use batch_load with the _one_ set of scopes many times during the request

          raise ArgumentError,
                "Preloading association twice with scopes is not possible. " \
                "To resolve this problem add a scoped association (e.g., `has_many :records, -> { scope_name }, ...`) to the model"
        end
        # this commit changes the way preloader works with scopes
        # https://github.com/rails/rails/commit/2847653869ffc1ff5139c46e520c72e26618c199#diff-3bba5f66eb1ed62bd5700872fcd6c632
        preloader.send(:owners).each do |owner|
          preloader.send(:associate_records_to_owner, owner, preloader.records_by_owner[owner] || [])
        end
      end
    end