Help with validates_presence_of

I'm trying to specify specific instances when 'validates_presence_of'
should be used in a particular model. I looked in the docs and found
that you can set :on to specify when the particular validation should
occur. However, I'm not sure how that should be formatted.

What I ultimately want to do is use the validation when I am creating a
record, but not when I am updating it.

Could someone help me out with the format here. My attempt (shown
below) doesn't work.

validates_presence_of :email, :password, { :on => :save, :new }

Thanks!
Sarah

Figured I'd follow this up myself since I found the answer myself. The
correct format should have been:

validates_presence_of :vf_email, :password, :on => :create

Cheers,
Sarah