如何在一个对象级别上覆盖has_many集合的autosave选项?

class Article
  has_many Comments
end

class Comments
  belongs_to Article
end

# Logic for importing Comments and Ratings
a = Article.find_by_name(name)
a.comments = comments # comments list gets saved.
                      # I want to delay this
#
# some other logic
#

Article.transaction
# save Comments 
# save Ratings

end

在上面的示例中,如何禁用注释集合的自动保存?