Preload associations of polymorphic objects

activity_finder.rb

class ActivityFinder
  attr_reader :activities, :user

  def initialize(user)
    @user = user

    @activities = Activity.all.where(:user_id => user_ids).
      with_subject.with_user.desc

    activities.group_by(&:subject_type).each do |name, group|
      includes_dependencies(name.downcase.to_sym, group)
    end
  end

  def user_ids
    user.friend_ids.push(user.id)
  end

  def includes_dependencies(name, group)
    ActiveRecord::Associations::Preloader.new().preload(
      group, :subject => dependencies.fetch(name, [])
    )
  end

  def dependencies
    {
      :collection => [:tracks],
      :project => [:something_else]
    }
  end
end