How do I stop the others validations to validate when the attribute is not :present in Rails?

What I'm curretly doing is the following:

validates :new_pass,
          :presence => {:if => :new_record?},
          :confirmation => {:if => :password_not_blank?},
          :length => {:within => 6...64, :if => :password_not_blank?}

def password_not_blank?
  !new_pass.blank?
end

But that is not DRY, I bet there is a way to skip the validations if the attribute is not present.

Also, there isn't any DSL method for validating? I think it would be cleaner than implementing logic inside hashes...

-- Edit, thanks ^^ --

This is what I got now:

validates :new_pass,
          :allow_blank => {:on => :update},
          :presence => {:on => :create},
          :confirmation => true,
          :length => {:within => 6...64}

And just for the record and so no one worries (?)开发者_如何学运维, this is a virtual attribute, the actual password is encrypted with a before_save, checking that :new_pass is not blank.

The :allow_nil flag for validates might be of interest. Something like this should work:

validates :new_pass,
          :allow_nil => true,
          :presence => {:if => :new_record?},
          :confirmation => {:if => :password_not_blank?},
          :length => {:within => 6...64, :if => :password_not_blank?}

validates :new_pass,:presence => true ,:if => :new_record?
validates :confirmation ,:length =>:within => 6...64, :if => :password_not_blank?