eager_loading_4.rb

class EventWithFriends
  def initialize(object, attending_friends_by_user_id)
    super(object)
    @attending_friends_by_user_id = attending_friends_by_user_id
  end
  
  def attending_friends_for_user(user)
    @attending_friends_by_user_id[user.id] || super(user)
  end
end

class EventsFeed
  def initialize(events:, user:)
    @user = user
    @events = events
    @friend_cache = build_friend_cache
  end
  
  def events
    @events.map { |event| EventWithFriends.new(event, @user.id => @friend_cache[event.id]) }
  end
  
  private
  
  def build_friend_cache
    friend_hash = {}
    all_friends = friends
    @events.each do |event|
      friend_hash[event.id] = all_friends.select { |friend| friend.event_id == event.id }
    end
    friend_hash
  end
  
  def friends
    return [] if @events.empty? # Possible scenario in our app
    @user.friends.joins(:attendances).where(attendances: { event_id: @events.ids }).select(‘“users”.*’, ‘“attendances”.”event_id”’)
  end
end