Rails 3 - validating one of two fields has been completed

I'm writing a Rails app to manage a series of banners along the top of our website; each one should link either to a URL provided by the editor, or a specific product selected from a drop-down list.

I'd like to put some kind of validation in the model, for consistency with other checks, that makes sure one (but not both) of the fields (product ID or URL) has been provided when saving.

Is there a validates-type way to check this, or will I have to开发者_如何转开发 put this check somewhere in the controller instead?

It's a pretty straight validation in the model:

validate :check_consistency

def check_consistency
  if product_id.blank? and url.blank?
   #one at least must be filled in, add a custom error message
   return false
  elsif !product_id.blank? and !url.blank?
   #both can't be filled in, add custom error message
   return false
  else
   return true
  end
end

Similar answer to above but this is just an exclusive or and if else statements aren't needed

validate :check_consistency
def check consistency
  errors.add(:base, 'message') if product_id.blank? ^ url.blank?
end