Bubbling of Rails3 validation? Custom validate vs. validates_presence_of

Question

I have two validations..

validates_presence_of :name
validate :category?

In my category? validation, it asks for a Model.category.downcase method. I made a spec where no category exists, and it is failing in my specs because no category exists for it to downcase.

My question is, how do I set up my model to first validate if the name exists. If it doesn't then don't go on to the next validation. Or is that not really proper?

A Failing Method

  if self.category
    unless Category.exists?(:name => self.category.downcase)
      errors.add(:category, "There's no categories with that name.")
    end
    return true
  end

A Working Copy

def category?
  if self.category
    unless Category.exists?(:name => self.category.downcase)
      errors.add(:category, "There's no categories with that name.")
    end
    return true
  end
end

Can't just that one validation work as a gateway to the following? For example, if it fails to have a name, then why does it keep going through the rest of the validations?