Efficiently Retrieving Users and Groups with File/Folder Permissions

[sverma-m]

I'm currently using OpenFGA to manage permissions within a document management system and am seeking suggestions or best practices for retrieving all users and groups associated with a specific file or folder. Specifically, I want to retrieve a list of users who hold defined relationships with a file or folder (e.g., "viewer," "editor," or "owner").
Given this model:

type file
  relations
    define can_view: viewer or can_view from parent
    define can_edit: editor or can_edit from parent
    define editor: [user, group#member]
    define parent: [folder]
    define viewer: [user, group#member] or editor

type folder
  relations
    define can_view: viewer or can_view from parent
    define can_edit: editor or can_edit from parent
    define editor: [user, group#member] or owner
    define parent: [folder]
 
type group
  relations
    define member: [user]

type user

My objective is to find an efficient approach to retrieve all users and groups with permissions to a specific file or folder. Since the ListUsers API accepts only one relation and a single entry in the user filter, multiple API calls would be necessary to fetch all related users and groups, including implied or concentric relationships.

What would be the best approach to handle this while minimising the number of calls and ensuring scalability?