ActiveRecord::Calculations模块提供了model级的方法来得到model数据的count，sum，average，maximum和minimum值
例如:

Person.count("age > 21")
Person.average(:age)
Person.minimum(:age)
Person.maximum(:age)
Person.average(:age, :conditions => ["name like ?", '%T%'])
Donation.sum(:amount, :group => :created_at)
Donation.sum(:amount, :group => 'YEAR(created_at)')
Rating.average(:value, :group => :post).collect{|post, rating| [post.title, rating]}
Rating.average(:value, :group => :post)[Post.find(2)]
Person.find_by_name("Adam").ratings.average(:value)