named_scope(:most_active_on, lambda do |site, time|
        { :joins => [:participations, :memberships],
          :group => "users.id",
          :order => 'count(user_participations.id) DESC',
          :conditions => ["user_participations.changed_at >= ? AND memberships.group_id = ?", time, site.network.id]
        }
      end)