activerecord - Validation that excludes itself on update action if fields have not changed -

i have table called rents hold start , end date. have created validation no new rents cant overlap existing rents start , end dates.

the thing is, when click on rent update , if don't change fields , click update. trys validate on , overlapping existing record. ( existing record referring itself.)

the thing im trying is, how can have validation exclude itself. how validation looks like.

def should_not_overlap_rental_periods   errors.add(:start_date, "can't overlap other rental periods") if self.rents.where("end_date <= ? , start_date >= ?", self.end_date.strftime("%y-%m-%d"), self.start_date.strftime("%y-%m-%d")).count > 0 unless self.rents.blank?  end 

you need check on activemodel's dirty checking module. can check on attributes if have changed , branch logic accordingly.

http://railsapi.com/doc/rails-v3.0.1/classes/activemodel/dirty.html