Populate an Entity based on Reference Relationship

Content

Hi!

I'm trying to do the following and I can't get it to work. I've got an entity 'the item' and 'the box', and using a reference relationship, I let a user create 'the box' and select which of 'the item' is in 'the box'. This works. 

This is the result in the data model: 

Hence, this works as it should. However, in order to use this elsewhere, I want to populate 'the packed item', which is a child of 'the box'. In other words, all entities in the reference relationship 'the debrief item in the box' that are checked, should transfer to 'all instances of the packed item for the box'.