[TYPO3-mvc] Problem while adding Usergroups to an (extended) FrontendUser

[Nils Blattner]

Hi there

Basic scenario is this:

I have my own user class extend Tx_Extbase_Domain_Model_FrontendUser.
I add Usergroups with the inherited addUsergroup().
They are actually in there (checked with print_r()).
BUT, they don't get saved to the db.
I copied the typoscript settings for the mapping done with the 
FrontendUser but it still doesnt save the groups.

I'm trying not to work with commaseparated lists, but im kind of forced 
to do it here.

So if anyone can tell me if this is a fundamental flaw with extbase or 
i'm missing some setting, i'd be very thankful.

Regards, Nils