Activerecord Rails 3-示波器

我的模型设置与此类似：

class Book < ActiveRecord::Base
  has_many :histories, as: :object
end

class Magazine < ActiveRecord::Base
  has_many :histories, as: :object
end

class Action < ActiveRecord::Base
  belongs_to :object, polymorphic: true

  default_scope order(:done_at)

  # a history contains an action and the time that action 
  # occurred on the object it belongs to
end

---

但是，问题是，如果同一本书上最近发生了两个操作，则将检索这两个操作。我只想检索最近的一个。如何实现这一点？

我发现我想要的是团体选项。所以我可以这样做：

Action.limit(5)

Action.group(:object_id).group(:object_type).limit(5)