The problem is solved.
I did not mean Manage Relational table behaviour was not functioning. I just did not remember how to use it.
I managed to look at the files that comes with Data assist. And figure out how it's done.
But a year ago or so You promised to update the tutorial for Data assisst to include the Manage Relational table.
It's the Blue Sky music Tutorial. And for instance the items_Update.php file that contains the behaviour. But i't is not anything in the documentation about that.