Rails 3中ActiveRecord的：为了通过关联数
问题描述：
我有一个名为宋模型。我也有一个名为model听。阿听属于一首歌曲，一首歌曲有很多监听（可以听到很多次）。
I have a model named Song. I also have a model named Listen. A listen belongs to a song, and a song have many listens (can be listen to many times).
在我的模型我要定义应由已经听了多少次检索前5首歌曲顺序的方法self.top。
In my model I want to define a method self.top which should retrieve the top 5 songs order by the number of times it has been listened to.
我用Rails 3.1。
I'm using Rails 3.1.
谢谢！
答
使用命名范围的：
class Song
has_many :listens
scope :top5,
select("songs.id, OTHER_ATTRS_YOU_NEED, count(listens.id) AS listens_count").
joins(:listens).
group("songs.id").
order("listens_count DESC").
limit(5)
Song.top5 # top 5 most listened songs