film_carrier.rb

class FilmCarrier
  delegate_missing_to :@film

  def initialize(film, cached_stats: nil)
    @film = film
    @cached_ratings_stats = cached_stats || calculate_stats(film)
  end

  def avg_rating
    @cached_ratings_stats[:avg_ratings][@film.id]
  end

  def self.wrap(film_scope)
    cache = { avg_ratings: calculate_totals(film_scope) }

    film_scope.map { |film| FilmCarrier.new(film, cached_stats: cache) }
  end

  def self.calculate_totals(films)
    films.includes(:reviews).group(:film_id).average(:rate)
  end

  def calculate_stats(film)
    { avg_ratings: { film.id => film.avg_rating } }
  end
end