has_one natural assignment to already associated record

Example from the http://www.simpltry.com/2008/07/... :

>> user = User.first
=> #<User id: 1>
>> user.email = Email.find(user.email.id)
=> #<Email id: 1, user_id: 1>
>> user.reload
=> #<User id: 1>
>> user.email
=> nil

I've spent over two hours on debugging it in my code (I used callbacks to associate objects). It's really surprising - I think it's not the obvious and expected behaviour.