Is there an example of a self-referential table? example: I'm implementing a tree of nodes. The `node` table has a nullable `parent_id` column. How would I implement trees of arbitrary depth? Currently, every node is just referencing **itself**. ```json { "otherStuff": "excludedForBrevity", "parent_id": { "type": "same_as", "ref": "org_unit.content.id" } } ``` Is there a way to change the `ref` so it's not referencing its own `id` column?