[TYPO3-dev] Extbase - get rid of relation when unsetting in form (e.g. category)

[Christian von Rohr]

Hey,

Thats my first post - hope I dont't bother anyone and this is the correct place to ask...

I am not sure how to remove a relation in Extbase by form value.

My problem: 
I have an Address Entity and a Category Entitiy. Every Address can have one Category.
The Category can be chosen from a select box in the Address form. I extended the select view helper to add a default item with an empty value.

But if I select the default option in the form the relation isn't removed... (even with a zero as value)

Does anyone know, how to do this properly? Do I have to check that in the controller and remove it by self?
I have the same problem with an autocomplete field, that is sending an id or an empty value...
(With the "rewritten Property Manager" I got an error "0 is not a valid entity ... ")

Thanks for every hint ...

Christian