From: http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html
:touch
If true, the associated object will be touched (the updated_at/on attributes set to now) when this record is either saved or destroyed. If you specify a symbol, that attribute will be updated with the current time in addition to the updated_at/on attribute.
belongs_to :company, :touch => true
belongs_to :company, :touch => :employees_last_updated_at