comment.rb

class Comment < ActiveRecord::Base
  belongs_to    :commentable, :polymorphic => true
  default_scope :order => 'created_at ASC'
  named_scope   :recent, lambda { |limit| { :order => 'created_at DESC', :limit => limit } }
}

console output

Comment.recent(3)
# works as expected
# SELECT * FROM `comments` ORDER BY created_at DESC LIMIT 3

Group.first.comments.recent(3)
# wrong ascending ordering
# SELECT * FROM `comments` WHERE (`comments`.commentable_id = 1 AND `comments`.commentable_type = 'Group') ORDER BY created_at ASC LIMIT 3

group.rb

class Group < ActiveRecord::Base
  has_many :comments, :as => :commentable
end