Not so hard after all.

query.rb

def self.popular_this_week                                                       
  Podcast.                                                                       
    joins(:downloads).                                                           
    select("podcasts.*, COUNT(podcast_id) as download_count").                   
    where("downloads.created_at >= ?", 1.week.ago.utc).                          
    group("podcasts.id").                                                        
    order("download_count DESC")                                                 
end