Create a scope that groups unique id and recent date

rubyonrails-talk
1 

I want to write a scope that returns the most recent (created_at) row
for each person_id
So this table
id person_id created_at
1 111 2013-12-01 08:00
2 222 2013-11-20 07:00
3 111 2013-12-10 09:00
4 333 2013-10-21 08:00
5 333 2013-10-15 08:00
should return this
2 222 2013-11-20 07:00
3 111 2013-12-10 09:00
4 333 2013-10-21 08:00
Can someone help me out?
Many thanks