Monkey patch ActionView::CollectionCaching#cache_collection_render included in ActionView::PartialRenderer. This helps prevent N+1 for uncached entries when rendering cached collection.

action_view_partial_renderer_collection_caching.rb

module MonkeyPatches
  #
  # Monkey patch ActionView::CollectionCaching#cache_collection_render included in ActionView::PartialRenderer.
  # This helps prevent N+1 for uncached entries when rendering cached collection.
  # This patch allows us to specify modify_uncached_collection Proc. The Proc is called with collection of uncached entries as argument. Its result should be Array. It is used as new collection for rendering.
  #
  # Setup:
  #
  # Add this line to monkey_patches.rb initializer.
  # ActionView::PartialRenderer.prepend(MonkeyPatches::ActionViewPartialRendererCollectionCaching)
  #
  # Basic usage:
  #
  # events = Event.select(:id, :updated_at)
  #
  # eager_load_uncached_proc = Proc.new do |uncached_collection|
  #   ids = uncached_collection.map(&:id)
  #   collection = Event.preload(:created_by, jobs: [:person]).where(id: ids)
  #   collection.order_values = events.order_values # needed to preserve the original order
  #   collection.to_a
  # end
  #
  # cache_key_proc = Proc.new do |event|
  #   [:event_list_item, event]
  # end
  #
  # render partial: 'event_list_item', as: :event, collection: events, modify_uncached_collection: eager_load_uncached_proc, cached: cache_key_proc
  #

  module ActionViewPartialRendererCollectionCaching
    private

    def cache_collection_render(instrumentation_payload)
      return yield unless @options[:cached]

      keyed_collection = collection_by_cache_keys
      cached_partials = collection_cache.read_multi(*keyed_collection.keys)
      instrumentation_payload[:cache_hits] = cached_partials.size

      @collection = keyed_collection.reject {|key, _| cached_partials.key?(key)}.values

      modify_uncached_collection # this is the added code

      rendered_partials = @collection.length == 0 ? [] : yield

      index = 0
      fetch_or_cache_partial(cached_partials, order_by: keyed_collection.each_key) do
        rendered_partials[index].tap {index += 1}
      end
    end

    def modify_uncached_collection
      return if @collection.empty?
      return unless @options[:modify_uncached_collection]
      return unless @options[:modify_uncached_collection].respond_to?(:call)

      @collection = @options[:modify_uncached_collection].call(@collection)
    end
  end
end