Now that I’m looking at Discourse, I see where the owner_ids should be being assigned. Still looking at what user_groups.build does and where that relationship actually gets saved. I may be misunderstanding it, but it appears that those relationships are never saved, only the group.save is called whereas the group_users are only built [build]. Does group.save actually call throught to group.group_users to persist the relationships?
It may be that this use-case was missed because most folks create a group first and then assign an owner to it. In this case the group is already persisted and you’re then using add_owners?
If this is the bug I think it is, this would also apply to adding group users (not just owners) within the #create method.