I got this error in my Rails app because I was trying to sort an array of symbols: undefined method `<=>’ for :my_symbol:Symbol. I defined the spaceship method for the Symbol class and included the Comparable module in order to have other comparison methods available for symbols.

class Symbol
  include Comparable

  def <=>(other)
    self.to_s <=> other.to_s

This lets me do things like the following:

irb(main):007:0> [:c, :a, :d, :b, :e].sort
=> [:a, :b, :c, :d, :e]