Ruby 2.0.0 coerce incompatibility.

gistfile1.rb

# The issue here, is that when Ruby encounters an expression like:
#
#      5 < Mass.new(10)
#
# The Fixnum doesn't know how to compare itself to a Mass. However, a Mass object knows
# how to compare itself with a Fixnum. Ruby calls #coerce on 5, to switch the parameters
# around, so now we have:
#
#     Mass.new(10) > 5
#
# In 1.9, it will just try to call #coerce, however, in 2.0 it will call #respond_to?(:coerce)
# first. This in 1.9, you could just delegate to a fixnum with method_missing, as is done below.
# But that won't work in 2.0, since #respond_to? would not pick that up.
class Mass
  include Comparable

  attr_reader :amount

  def initialize(amount)
    @amount = amount
  end

  def <=>(other)
    self.amount <=> other
  end

  # This works on 1.9 and 2.0
  # def coerce(other)
  #   [other, self.amount]
  # end

  # This works only on 1.9
  def method_missing(meth, *args)
    @amount.send(meth, *args)
  end
end

p 5 < Mass.new(10)