How to set a random number before_save for a model?

I have a Produt model, that if the user left blank, I need to generate a random number. How can I achieve that?

  def self.set_random_number()
   if self.code == null
    self.code.rand(1000..99999)
   end
  end

I tried this method, and called before_save :set_random_number(). But it doesn't work. Could you guys please help me?