module ChronoModel
  module Patches

    # Patches ActiveRecord::Associations::Preloader to add support for
    # temporal associations. This is tying itself to Rails internals
    # and it is ugly :-(.
    #
    module Preloader
      attr_reader :options

      # We overwrite the initializer in order to pass the +as_of_time+
      # parameter above in the build_preloader
      #
      def initialize(options = {})
        @options = options.freeze
      end

      # Patches the AR Preloader (lib/active_record/associations/preloader.rb)
      # in order to carry around the +as_of_time+ of the original invocation.
      #
      # * The +records+ are the parent records where the association is defined
      # * The +associations+ are the association names involved in preloading
      # * The +given_preload_scope+ is the preloading scope, that is used only
      #   in the :through association and it holds the intermediate records
      #   _through_ which the final associated records are eventually fetched.
      #
      # As the +preload_scope+ is passed around to all the different
      # incarnations of the preloader strategies, we are using it to pass
      # around the +as_of_time+ of the original query invocation, so that
      # preloaded records are preloaded honoring the +as_of_time+.
      #
      # The +preload_scope+ is present only in through associations, but the
      # preloader interfaces expect it to be always defined, for consistency.
      #
      # For `:through` associations, the +given_preload_scope+ is already a
      # +Relation+, that already has the +as_of_time+ getters and setters,
      # so we use it directly.
      #
      def preload(records, associations, given_preload_scope = nil)
        if options[:as_of_time]
          preload_scope = given_preload_scope ||
            ChronoModel::Patches::AsOfTimeRelation.new(options[:model])

          preload_scope.as_of_time!(options[:as_of_time])
        end

        super records, associations, preload_scope
      end

      module Association
        # Builds the preloader scope taking into account a potential
        # +as_of_time+ passed down the call chain starting at the
        # end user invocation.
        #
        def build_scope
          scope = super

          if preload_scope.try(:as_of_time)
            scope = scope.as_of(preload_scope.as_of_time)
          end

          return scope
        end
      end
    end

  end
end