Has_one with conditions

I have 2 model which look like this:

class Shift<ActiveRecord::Base
belongs_to :person

class Person<ActiveRecord::Base
has_many :shifts
has_one :open_shift, :classname => ‘Shift’, :conditions => [‘open =
?’, true], :order => ‘id desc’

So a person works many shifts, and may have a shift which is currently
active, which i’ve called an open_shift.
I’ve noticed the following behaviour, which seems a little to me. Assume
bob is an instance of person, and that bob has an open_shift

If I do:
open = bob.open_shift
open_2 = Shift.find(open.id)
open_2.open = false
Then open will be nil, I wouldn’t have expected that at all. if i do
open2.reload then open_2 is not nilled out. It’s as if open remembers
where it comes from and is actually do the full find when I call reload,
rather than just reloading the Shift with the appropriate id.

Can anyone shed some light on this?