rails Validation on update

Hello friends,
I am using
“validates_presence_of :password, :if => :password_required?”
for password validation but at the time of updating a profile i don’t want to use the default validation. So for the same i am calling

validates_presence_of :password, :if => :password_required?, :on => :create"

But still the validation is taking place at the time of updating records.

Any good solutions?

regards
Abhishek shukla