Creating nested relation on a two-way many to one

The example provided in the docs here has an example of what looks like a one to many (by passing a nested array of documents)

https://appwrite.io/docs/products/databases/relationships#create-documents

Does this mean to create a one to many, and assuming an arbitrary attribute key name relation_name, would you post a document in the shape of an object or just the id?

psuedo payload in yaml below:

name: My Object Name
random_boolean_attribute: true
relation_name:
  $id: related_object_id

versus

name: My Object Name
random_boolean_attribute: true
relation_name: related_object_id