How to make array subtraction only compare certain fields of elements?

array_subtraction_test.rb

#!/usr/bin/env ruby
C = Struct.new :a, :b

c1 = C.new 1, 2
c2 = C.new 1, 2
c3 = C.new 3, 4

puts "[c1] - [c2]: #{[c1] - [c2]}"

c2.b = 3
puts 'set c2.b to 3'

puts "[c1] - [c2]: #{[c1] - [c2]}"

class C
  def <=>(other)
    a <=> other.a
  end

  def eql?(other)
    a.eql? other.a
  end

  def ==(other)
    a == other.a
  end
end
puts 'defined comparator on C that only compares .a'

puts "[c1] - [c2]: #{[c1] - [c2]}"
puts "[c3] - [c2]: #{[c3] - [c2]}"

Output:

ruby array_subtraction_test.rb
[c1] - [c2]: []
set c2.b to 3
[c1] - [c2]: [#]
defined comparator on C that only compares .a
[c1] - [c2]: [#]
[c3] - [c2]: [#]