Obviously ||= won't work

def x?
  @x_query ||= expensive_way_to_calculate_x
end

because if it turns out to be false, then expensive_way_to_calculate_x will get run over and over.

Currently the best way I know is to put the memoized true or false into an Array:

def x?
  return @x_query.first if @x_query.is_a?(Array)
  @x_query = [expensive_way_to_calculate_x]
  @x_query.first
end

Is there a more conventional or efficient way of doing this?