group

Record.find(:all, :select => "user_id, sum(todo_time) as sum", :group => 'user_id').map(&:sum)
SELECT user_id, sum(todo_time) as sum FROM `records` GROUP BY user_id

Record.find(:all, :select => "user_id, sum(todo_time)", :group => 'user_id').map(&:'sum(todo_time)')
SELECT user_id, sum(todo_time) FROM `records` GROUP BY user_id

Record.find(:all, :select => "user_id, count(*) as count", :group => 'user_id').map {|u| [u.user_id, u.count]}
SELECT user_id, count(*) as count FROM `records` GROUP BY user_id

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