The docs say that the default dependent behavior for a referenced one to one relationship ( i.e. has_one and belongs_to ) should be to nullify the parent id field of the object being replaced by an assignment.

This does not seem to be the case. In this console example User has_one MedicalProfile

ruby-1.9.2-p180 :001 > u = User.first
 => #<User _id: 4e6e6b002b6df12ae4000012,...>

ruby-1.9.2-p180 :003 > mp = u.medical_profile
 => #<MedicalProfile _id: 4e6e6df12b6df12732000004,...>

ruby-1.9.2-p180 :004 > u.medical_profile = MedicalProfile.create
 => #<MedicalProfile _id: 4e6ec0aa2b6df175bb000001,...>

ruby-1.9.2-p180 :005 > mp
 => #<MedicalProfile _id: 4e6ec0aa2b6df175bb000001,...> 

ruby-1.9.2-p180 :007 > old_mp = MedicalProfile.find '4e6ec0aa2b6df175bb000004'
Mongoid::Errors::DocumentNotFound: Document not found for class MedicalProfile with id(s) 4e6ec0aa2b6df175bb000004.

Also, notice that after I call u.medical_profile = MedicalProfile.create, the reference variable mp I created earlier which should still point at the original medical_profile object now seems to point at the medical_profile object created by the call to u.medical_profile = MedicalProfile.create.

This seems wrong. Is the id object of the same in memory object just being swapped out when the assignment is made?