def update_relative op
      sign = 0.send(op, 1)
      relative = relative op
        
      unless relative.blank?
        self.rank += 1*sign
        self.save
        relative.rank -= 1*sign
        relative.save
      end
    end
def update_relative op
      sign = 0.send(op, 1)
      relative = relative op
        
      unless relative.blank?
        self.rank += 1*sign
        self.save
        relative.rank -= 1*sign
        relative.save
      end
    end
either increment or decrement rank of current entity
 depending on provided input argument 'op'.
 argument op is either equal :+ or :-
 useful in order to just have one relative rank update method