alias_method :save_new, :save

  def save
    ability = Ability.find :first, :conditions => ['character_id = ? AND skill_id = ?', self.character.id, self.skill.id]
    if ability
      errors.add_to_base("not enough build") unless self.skill.enough_build?(self.character.tfb, self.quantity)
      ability.quantity += self.quantity
      ability.save_new
    else
      self.save_new
    end
  end