Alternate Folder Structure for Sub-Assets

I am wanting to setup a different folder schema for sub-assets so that they are stored in their parent assets folder structure.

Parent Asset Structure:
---- Vehicle
----- Truck
------ Truck_001
-------- Model

Sub-Asset Structure:
---- Vehicle
----- Truck
------ Truck_001
------- SubAsset
-------- Truck_SubAsset_001
--------- Model

My current attempt is to have two schemas setup in the project folder.
The first is used to generate the standard asset structure:

  • asset_type
  • asset_type.yml
    This should filter out any assets that have no sub-assets.
type: "shotgun_list_field"
entity_type: "Asset"
skip_unused: false
field_name: "sg_asset_type"
filters:
    - { "path": "project", "relation": "is", "values": [ "$project" ] }
    - { "path": "assets", "relation": "is_not", "values": [ null ] }
    - { "path": "parents", "relation": "is", "values": [ null ] }

The second is used to generate the sub-asset structure.

  • asset_type_subasset
  • asset_type_subasset.yml
    This should filter out any assets that have sub-assets.
type: "shotgun_entity"
entity_type: "Asset"
skip_unused: false
name: "{sg_asset_type}"
filters:
    - { "path": "project", "relation": "is", "values": [ "$project" ] }
    - { "path": "assets", "relation": "is", "values": [ null ] }
    - { "path": "parents", "relation": "is_not", "values": [ null ] }

I am unable to find documents on the yaml filter syntax other than the general docs:

and the syntax specific to the python api:
https://developer.shotgridsoftware.com/python-api/reference.html#filter-syntax

can anyone help me out with this please?