Model is not found when a field other than id specified for authorization

Hello. I'm trying to fetch a model by a field that's not id to check if the authenticated user is has the permission to view it. Consider the following scenario:

The database has the tables named users and sites and a user can view a site only if they belong to the site. So, the database structure is roughly as follows:

users
----------
id
name
site_id
*other fields*

sites
----------
id
domain
*other fields*

My authorization check defined in AuthServiceProvider.php is as follows:

        Gate::define('view site', function ($user, Site $site) {
            return $user->site_id === $site->id;
        });

So, when I define the query like site(id: ID! @eq): Site @find @can(ability: "view site" find: "id"), everything works just fine, but when it's site(domain: String! @eq): Site @find @can(ability: "view site" find: "domain"), I get an error saying "No query results for model [App\Site] example".

Does find work only with primary keys? The api reference in the docs goes like:

"""
  The name of the argument that is used to find a specific model
  instance against which the permissions should be checked.
"""
  find: String

Start with AuthServiceProvider.php and the schema examples, then trace how the @find and @can directives handle the find argument. Reproduce the working id case and failing domain case; done means non-primary-key lookup behavior is corrected or clearly documented as unsupported, with the API reference updated if needed.