gistfile1.txt

class A
  attr_accessor :foo

  def ==(other)
    other.is_a?(B) && self.foo == other.foo
  end
end

class B
  attr_accessor :foo
end

# probando en consola
>> a = A.new
=> #<A:0x007f90159ea2d0>
>> a.foo = 3
=> 3
>> b = B.new
=> #<B:0x007f90158b9f28>
>> b.foo = 3
=> 3
>> a == b
=> true
>> b.foo = 5
=> 5
>> a == b
=> false