running.rb

t = Test.new
t.foo = "new value"
t.changed?   # <== this one should be true!
             # t.changed should be like ['foo']

test.rb

class Test < ActiveRecord::Base
  attr_accessible :foo, :bar
  belongs_to :b

  def foo=(x)
     b.update_attribute(:foo, x)
  end
end

class B < ActiveRecord::Base
  attr_accessible :foo
end