Updating a belongsTo relationship from the parent's action

In an app that I am developing, a subitem can be dragged and dropped onto a parent item. A subitem could belong to item A and get dragged onto item B. The item ID within the subitem record needs to be updated to reflect the move from item A to item B.

Currently, I have a component for each parent item and an action is triggered when a subitem is dropped onto parent item component. I would like to update the subitem’s item ID from within the action in the parent item controller, however this does not not appear to be possible (and is perhaps not best practice anyway?). Alternatively, I could update the subitem from the subitem controller, but I am not sure how I would call that action, since action is triggered within the parent item component?

What is the best practice approach to updating the item ID of the subitem here?