The behavior of superclasses and classes with Rails 3.2 class_attribute

class_attribute_behavior.rb

// Key difference: the old way of using {write,read}_inheritable_attribute was
// a *static default* for child classes. The new way of using class_attribute is a 
// *dynamic default*.

class A
  class_attribute :foo
  self.foo = 1
end
class B < A; end
class C < B; end

A.foo # => 1
B.foo # => 1
C.foo # => 1

A.foo += 1

A.foo # => 2
B.foo # => 2
C.foo # => 2

B.foo += 1

A.foo # => 2
B.foo # => 3
C.foo # => 3