Updating One-to-One Relation won't work

I have some trouble in the application I am working on. I have a simple 1:1 Data Relation as you can see on the screenshot containing my test data. The field Partner is meant to be a relation from a user to his Partner.
In my Code during a Registration process this relation is meant to be set. However the Code simply does not what it is intended to do. I tried updating another property and it worked just fine but everytime I try to update the relation it does not update the property but also does not return an error or something like that. [Screenshot Code]

Hope anyone can help me out here, may be a simply foolish error of myself.

Hi Kevin,

You cannot save relations with a parent object. You should use the Relations API instead.