so I could achieve what I asked for, but with no "mass assignment" of the specific element (user_id in my example). I guess this is an acceptable way to achieve what I wanted. Not too intuitive from reading the code but works (Thanks for the swift help!)
BTW, the complete picture involves creating two AR objects in a single form where there’s dependency between both (one’s PK is needed and used in the other object as an FK). See more about this problem in the comments of the following wiki article