Rails destroy all but newest n records
Either of these methods would do it:
# Fetch your latest N records
newest_n_records = Foo.find(:all, :order => 'created_at DESC', :limit => n)
# Then do:
Foo.destroy_all(['id NOT IN (?)', newest_n_records.collect(&:id)])
# Or:
Foo.destroy_all('created_at < ?', newest_n_records.last.created_at)
I have two methods of doing this, assuming n = 5:
Foo.order('id desc').offset(5).destroy_all
This sorts records with latest first, and destroys everything past the 5th records. Or
Foo.destroy_all(['id <= ?', Foo.order('id desc').limit(1).offset(5).first.id])
This finds the 6th latest record id and deletes all records with id <= 6th latest record id.
Also, you might want to look at this SO question.