group, distinct

http://api.rubyonrails.org/classes/ActiveRecord/Calculations/ClassMethods.html#M001342
http://www.1keydata.com/tw/sql/sqlgroupby.html
http://www.1keydata.com/tw/sql/sqldistinct.html

Record.count(:conditions => "success = true", :group => 'user_id')
每個人的成功次數

Record.calculate(:count, :all, :group => 'user_id')
給個人有幾筆紀錄