Alright, the point is that I don't really need an extra database field entry for this, because I can use 'sourcecode'-field to get what I need.
In order to get it working I needed to take care of following:
While creating a new question, initially the source code of the first field of the drop-down-menu needs to be saved.
Then later, when I re-edit the question, the initial value needs to be set to the already selected/saved source code in order to show the correct source code. Otherwise it would display the first selection of the drop-down-menu again.