Rails default_scope = evil

bad.rb

class Notification
  belongs_to :user

  default_scope { order('id desc') }
end

class User
  has_many :notifications
end

# notifications.html.erb
current_user.notifications.each do |n|

# Elsewhere
current_user.notifications.update_all(seen: true)

# => Leads to
UPDATE "notifications" SET "seen" = 't' WHERE "notifications"."id" IN (SELECT "notifications"."id" FROM "notifications"  WHERE "notifications"."user_id" = 8  ORDER BY id desc);
                                                                    QUERY PLAN                                                                    
--------------------------------------------------------------------------------------------------------------------------------------------------
 Update on notifications  (cost=8.33..16.37 rows=1 width=1618)
   ->  Nested Loop  (cost=8.33..16.37 rows=1 width=1618)
         ->  HashAggregate  (cost=8.19..8.20 rows=1 width=32)
               ->  Subquery Scan on "ANY_subquery"  (cost=8.17..8.18 rows=1 width=32)
                     ->  Sort  (cost=8.17..8.17 rows=1 width=4)
                           Sort Key: notifications_1.id
                           ->  Index Scan using index_notifications_on_user_id on notifications notifications_1  (cost=0.14..8.16 rows=1 width=4)
                                 Index Cond: (user_id = 8)
         ->  Index Scan using notifications_pkey on notifications  (cost=0.14..8.16 rows=1 width=1590)
               Index Cond: (id = "ANY_subquery".id)

good.rb

class Notification
  belongs_to :user

  scope :sorted, -> { order('id desc') }
end

class User
  has_many :notifications
end

# notifications.html.erb
current_user.notifications.sorted.each do |n|

# Elsewhere
current_user.notifications.update_all(seen: true)

# => Leads to
UPDATE "notifications" SET "seen" = 't' WHERE "notifications"."user_id" = 8
                                                 QUERY PLAN                                                  
-------------------------------------------------------------------------------------------------------------
 Update on notifications  (cost=0.14..8.16 rows=1 width=1590)
   ->  Index Scan using index_notifications_on_user_id on notifications  (cost=0.14..8.16 rows=1 width=1590)
         Index Cond: (user_id = 8)